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/shlex-6d46c8d4ff43dda3/invoked.timestamp

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

crates/donglora-cobs/fuzz/target/release/.fingerprint/shlex-6d46c8d4ff43dda3/lib-shlex

@@ -0,0 +1 @@
+6fc9b2c7c9d86f2c

crates/donglora-cobs/fuzz/target/release/.fingerprint/shlex-6d46c8d4ff43dda3/lib-shlex.json

@@ -0,0 +1 @@
+{"rustc":4542549483023249286,"features":"[\"default\", \"std\"]","declared_features":"[\"default\", \"std\"]","target":929485496544747924,"profile":5691290398283089708,"path":2783901580333741854,"deps":[],"local":[{"CheckDepInfo":{"dep_info":"release/.fingerprint/shlex-6d46c8d4ff43dda3/dep-lib-shlex","checksum":false}}],"rustflags":[],"config":8247474407144887393,"compile_kind":0}