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-a8b33c1adb14b39c/invoked.timestamp

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

crates/donglora-cobs/fuzz/target/release/.fingerprint/libc-a8b33c1adb14b39c/lib-libc

@@ -0,0 +1 @@
+7dbb74ea2a4aff16

crates/donglora-cobs/fuzz/target/release/.fingerprint/libc-a8b33c1adb14b39c/lib-libc.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":17682796336736096309,"profile":14164772475361225590,"path":17518599698233055907,"deps":[[17159683253194042242,"build_script_build",false,12809474353704815611]],"local":[{"CheckDepInfo":{"dep_info":"release/.fingerprint/libc-a8b33c1adb14b39c/dep-lib-libc","checksum":false}}],"rustflags":[],"config":8247474407144887393,"compile_kind":0}