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/libfuzzer-sys-1aa67406e873cefd/build-script-build-script-build

@@ -0,0 +1 @@
+d1919c80dbbe21d1

crates/donglora-cobs/fuzz/target/release/.fingerprint/libfuzzer-sys-1aa67406e873cefd/build-script-build-script-build.json

@@ -0,0 +1 @@
+{"rustc":4542549483023249286,"features":"[\"default\", \"link_libfuzzer\"]","declared_features":"[\"arbitrary-derive\", \"default\", \"link_libfuzzer\"]","target":17883862002600103897,"profile":5691290398283089708,"path":2467578377727249729,"deps":[[17357505374796345780,"cc",false,7232208144723462200]],"local":[{"CheckDepInfo":{"dep_info":"release/.fingerprint/libfuzzer-sys-1aa67406e873cefd/dep-build-script-build-script-build","checksum":false}}],"rustflags":[],"config":8247474407144887393,"compile_kind":0}

crates/donglora-cobs/fuzz/target/release/.fingerprint/libfuzzer-sys-1aa67406e873cefd/invoked.timestamp

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