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/.fingerprint/libc-9342b57ae0d6d291/run-build-script-build-script-build

@@ -0,0 +1 @@
+fb4b0da40e65c4b1

crates/donglora-cobs/fuzz/target/release/.fingerprint/libc-9342b57ae0d6d291/run-build-script-build-script-build.json

@@ -0,0 +1 @@
+{"rustc":4542549483023249286,"features":"","declared_features":"","target":0,"profile":0,"path":0,"deps":[[17159683253194042242,"build_script_build",false,13812864306637731461]],"local":[{"RerunIfChanged":{"output":"release/build/libc-9342b57ae0d6d291/output","paths":["build.rs"]}},{"RerunIfEnvChanged":{"var":"RUST_LIBC_UNSTABLE_FREEBSD_VERSION","val":null}},{"RerunIfEnvChanged":{"var":"RUST_LIBC_UNSTABLE_MUSL_V1_2_3","val":null}},{"RerunIfEnvChanged":{"var":"RUST_LIBC_UNSTABLE_LINUX_TIME_BITS64","val":null}},{"RerunIfEnvChanged":{"var":"RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS","val":null}},{"RerunIfEnvChanged":{"var":"RUST_LIBC_UNSTABLE_GNU_TIME_BITS","val":null}}],"rustflags":[],"config":0,"compile_kind":0}