fix: move build-std from .cargo/config.toml to Justfile (Xtensa only)

build-std=core,alloc was globally enabled in .cargo/config.toml, which
conflicted with cargo-fuzz's own -Zbuild-std, causing duplicate core
errors. Now only passed for Xtensa targets via the Justfile.

crates/donglora-cobs/fuzz/target/release/build/compiler_builtins-fbb0930458ce98a4/build_script_build-fbb0930458ce98a4.d

@@ -0,0 +1,6 @@
+/home/swaits/Code/lora-dongle/crates/donglora-cobs/fuzz/target/release/build/compiler_builtins-fbb0930458ce98a4/build_script_build-fbb0930458ce98a4.d: /home/swaits/.local/share/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/compiler-builtins/compiler-builtins/build.rs /home/swaits/.local/share/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/compiler-builtins/compiler-builtins/configure.rs
+
+/home/swaits/Code/lora-dongle/crates/donglora-cobs/fuzz/target/release/build/compiler_builtins-fbb0930458ce98a4/build_script_build-fbb0930458ce98a4: /home/swaits/.local/share/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/compiler-builtins/compiler-builtins/build.rs /home/swaits/.local/share/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/compiler-builtins/compiler-builtins/configure.rs
+
+/home/swaits/.local/share/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/compiler-builtins/compiler-builtins/build.rs:
+/home/swaits/.local/share/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/compiler-builtins/compiler-builtins/configure.rs:

crates/donglora-cobs/fuzz/target/release/build/libc-9342b57ae0d6d291/invoked.timestamp

@@ -0,0 +1 @@
+This file has an mtime of when this was started.

crates/donglora-cobs/fuzz/target/release/build/libc-9342b57ae0d6d291/output

@@ -0,0 +1,25 @@
+cargo:rerun-if-changed=build.rs
+cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_FREEBSD_VERSION
+cargo:rustc-cfg=freebsd12
+cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_MUSL_V1_2_3
+cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_LINUX_TIME_BITS64
+cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS
+cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_GNU_TIME_BITS
+cargo:rustc-check-cfg=cfg(emscripten_old_stat_abi)
+cargo:rustc-check-cfg=cfg(espidf_time32)
+cargo:rustc-check-cfg=cfg(freebsd10)
+cargo:rustc-check-cfg=cfg(freebsd11)
+cargo:rustc-check-cfg=cfg(freebsd12)
+cargo:rustc-check-cfg=cfg(freebsd13)
+cargo:rustc-check-cfg=cfg(freebsd14)
+cargo:rustc-check-cfg=cfg(freebsd15)
+cargo:rustc-check-cfg=cfg(gnu_file_offset_bits64)
+cargo:rustc-check-cfg=cfg(gnu_time_bits64)
+cargo:rustc-check-cfg=cfg(libc_deny_warnings)
+cargo:rustc-check-cfg=cfg(linux_time_bits64)
+cargo:rustc-check-cfg=cfg(musl_v1_2_3)
+cargo:rustc-check-cfg=cfg(musl32_time64)
+cargo:rustc-check-cfg=cfg(vxworks_lt_25_09)
+cargo:rustc-check-cfg=cfg(target_os,values("switch","aix","ohos","hurd","rtems","visionos","nuttx","cygwin","qurt"))
+cargo:rustc-check-cfg=cfg(target_env,values("illumos","wasi","aix","ohos","nto71_iosock","nto80"))
+cargo:rustc-check-cfg=cfg(target_arch,values("loongarch64","mips32r6","mips64r6","csky"))

crates/donglora-cobs/fuzz/target/release/build/libc-9342b57ae0d6d291/root-output

@@ -0,0 +1 @@
+/home/swaits/Code/lora-dongle/crates/donglora-cobs/fuzz/target/release/build/libc-9342b57ae0d6d291/out

crates/donglora-cobs/fuzz/target/release/build/libc-e638e17bfb5d324b/build_script_build-e638e17bfb5d324b.d

@@ -0,0 +1,5 @@
+/home/swaits/Code/lora-dongle/crates/donglora-cobs/fuzz/target/release/build/libc-e638e17bfb5d324b/build_script_build-e638e17bfb5d324b.d: /home/swaits/.local/share/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libc-0.2.183/build.rs
+
+/home/swaits/Code/lora-dongle/crates/donglora-cobs/fuzz/target/release/build/libc-e638e17bfb5d324b/build_script_build-e638e17bfb5d324b: /home/swaits/.local/share/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libc-0.2.183/build.rs
+
+/home/swaits/.local/share/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libc-0.2.183/build.rs:

crates/donglora-cobs/fuzz/target/release/build/libfuzzer-sys-1aa67406e873cefd/build_script_build-1aa67406e873cefd.d

@@ -0,0 +1,5 @@
+/home/swaits/Code/lora-dongle/crates/donglora-cobs/fuzz/target/release/build/libfuzzer-sys-1aa67406e873cefd/build_script_build-1aa67406e873cefd.d: /home/swaits/.local/share/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libfuzzer-sys-0.4.12/build.rs
+
+/home/swaits/Code/lora-dongle/crates/donglora-cobs/fuzz/target/release/build/libfuzzer-sys-1aa67406e873cefd/build_script_build-1aa67406e873cefd: /home/swaits/.local/share/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libfuzzer-sys-0.4.12/build.rs
+
+/home/swaits/.local/share/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libfuzzer-sys-0.4.12/build.rs: