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/jobserver-4fe8dd28cee50fe9/invoked.timestamp

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

crates/donglora-cobs/fuzz/target/release/.fingerprint/jobserver-4fe8dd28cee50fe9/lib-jobserver

@@ -0,0 +1 @@
+ba3cea4f8cb37c23

crates/donglora-cobs/fuzz/target/release/.fingerprint/jobserver-4fe8dd28cee50fe9/lib-jobserver.json

@@ -0,0 +1 @@
+{"rustc":4542549483023249286,"features":"[]","declared_features":"[]","target":15857469692476194146,"profile":5691290398283089708,"path":11585431617667986584,"deps":[[17159683253194042242,"libc",false,1657124736078232445]],"local":[{"CheckDepInfo":{"dep_info":"release/.fingerprint/jobserver-4fe8dd28cee50fe9/dep-lib-jobserver","checksum":false}}],"rustflags":[],"config":8247474407144887393,"compile_kind":0}