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/find-msvc-tools-c53847beecb19cb8/invoked.timestamp

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

crates/donglora-cobs/fuzz/target/release/.fingerprint/find-msvc-tools-c53847beecb19cb8/lib-find_msvc_tools

@@ -0,0 +1 @@
+540ae849c6bd8ac6

crates/donglora-cobs/fuzz/target/release/.fingerprint/find-msvc-tools-c53847beecb19cb8/lib-find_msvc_tools.json

@@ -0,0 +1 @@
+{"rustc":4542549483023249286,"features":"[]","declared_features":"[]","target":10620166500288925791,"profile":6468643870482765141,"path":6475020921618913982,"deps":[],"local":[{"CheckDepInfo":{"dep_info":"release/.fingerprint/find-msvc-tools-c53847beecb19cb8/dep-lib-find_msvc_tools","checksum":false}}],"rustflags":[],"config":8247474407144887393,"compile_kind":0}