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/cc-f7c482496001664c/invoked.timestamp

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

crates/donglora-cobs/fuzz/target/release/.fingerprint/cc-f7c482496001664c/lib-cc

@@ -0,0 +1 @@
+38c8ed5ffdf65d64

crates/donglora-cobs/fuzz/target/release/.fingerprint/cc-f7c482496001664c/lib-cc.json

@@ -0,0 +1 @@
+{"rustc":4542549483023249286,"features":"[\"parallel\"]","declared_features":"[\"jobserver\", \"parallel\"]","target":11042037588551934598,"profile":6468643870482765141,"path":15881103530066230032,"deps":[[8410525223747752176,"shlex",false,3202016221234121071],[9159843920629750842,"find_msvc_tools",false,14306455825636919892],[16589527331085190088,"jobserver",false,2557116103657143482],[17159683253194042242,"libc",false,1657124736078232445]],"local":[{"CheckDepInfo":{"dep_info":"release/.fingerprint/cc-f7c482496001664c/dep-lib-cc","checksum":false}}],"rustflags":[],"config":8247474407144887393,"compile_kind":0}