ci: pin fuzz jobs to x86_64-unknown-linux-gnu

cargo-fuzz defaults to x86_64-unknown-linux-musl on Linux so ASAN can
statically link, but dtolnay/rust-toolchain@nightly doesn't ship a
musl std and GH runners don't have it preinstalled — build failed with
'can't find crate for core' on byteorder. Pinning both fuzz build and
fuzz run to the gnu target uses the std that comes with the toolchain
and works out of the box.

.github/workflows/ci.yml

@@ -120,13 +120,16 @@ jobs:
           tool: cargo-fuzz
       - name: Build fuzz targets
         working-directory: fuzz
-        run: cargo +nightly fuzz build
+        # Pin to the gnu target — cargo-fuzz defaults to musl for ASAN
+        # static linking, but the nightly toolchain we install doesn't
+        # ship a musl std.
+        run: cargo +nightly fuzz build --target x86_64-unknown-linux-gnu
       - name: Smoke-run every target
         working-directory: fuzz
         run: |
           for target in parse_command parse_radio_config cobs_decoder_feed cobs_roundtrip command_roundtrip cobs_decoder_multi_frame cobs_chunk_invariant; do
             echo "── $target ──"
-            cargo +nightly fuzz run "$target" -- -max_total_time=60
+            cargo +nightly fuzz run "$target" --target x86_64-unknown-linux-gnu -- -max_total_time=60
           done
 
   mutants: