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.

.cargo/config.toml

@@ -4,6 +4,3 @@ rustflags = [
     "-C", "link-arg=-Wl,-Tdefmt.x",
     "-C", "link-arg=-nostartfiles",
 ]
-
-[unstable]
-build-std = ["alloc", "core"]