fix(ci): give fmt/clippy/test distinct CARGO_TARGET_DIR to avoid races

After the candle deps were added, cargo builds run long enough that
the parallel fmt/clippy/test jobs (all on the `rust` runner label,
which appears to use act in host-executor mode) start racing each
other's intermediate temp files under
  /root/.cache/act/<hash>/hostexecutor/target/debug/deps/

Concretely the test job hit:
  error: No such file or directory at path
  "target/debug/deps/.tmprlicL7"
  Compiling unicode-ident
because another job's cargo invocation cleaned up the temp file
mid-compile. fmt and clippy happened to finish without their own
target races landing fatally, so only test failed visibly.

Set CARGO_TARGET_DIR=target-${{ github.job }} at the workflow level
so each job writes to its own target directory. sccache still backs
the actual rustc cache, so the rebuild penalty is just metadata not
full recompiles.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -16,6 +16,13 @@ env:
   SCCACHE_S3_USE_SSL: "false"
   AWS_ACCESS_KEY_ID: ${{ secrets.SCCACHE_S3_ACCESS_KEY }}
   AWS_SECRET_ACCESS_KEY: ${{ secrets.SCCACHE_S3_SECRET_KEY }}
+  # fmt, clippy, and test all run in parallel on the same `rust` runner
+  # and would otherwise share /root/.cache/act/<hash>/hostexecutor/target/,
+  # racing each other's cargo temp files (.tmpXXXXXX) and failing builds
+  # mid-compile. Give each job its own target directory so the invocations
+  # don't collide. sccache still backs the actual rustc cache, so the
+  # rebuild penalty is small.
+  CARGO_TARGET_DIR: target-${{ github.job }}
 
 jobs:
   fmt: