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-e638e17bfb5d324b/build-script-build-script-build

@@ -0,0 +1 @@
+8576d037e726b1bf

crates/donglora-cobs/fuzz/target/release/.fingerprint/libc-e638e17bfb5d324b/build-script-build-script-build.json

@@ -0,0 +1 @@
+{"rustc":4542549483023249286,"features":"[\"default\", \"std\"]","declared_features":"[\"align\", \"const-extern-fn\", \"default\", \"extra_traits\", \"rustc-dep-of-std\", \"rustc-std-workspace-core\", \"std\", \"use_std\"]","target":5408242616063297496,"profile":14164772475361225590,"path":16029548396519515336,"deps":[],"local":[{"CheckDepInfo":{"dep_info":"release/.fingerprint/libc-e638e17bfb5d324b/dep-build-script-build-script-build","checksum":false}}],"rustflags":[],"config":8247474407144887393,"compile_kind":0}

crates/donglora-cobs/fuzz/target/release/.fingerprint/libc-e638e17bfb5d324b/invoked.timestamp

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