ci: HF_HOME cannot be set from the job-level env block

195d60b never ran: GitHub rejected the workflow file itself, before any job
started, so the whole matrix reported failure with no logs.

Cause: `HF_HOME: ${{ runner.temp }}/hf-cache` sat in jobs.test.env, and the
`runner` context is not available there — only github / inputs / matrix /
needs / secrets / strategy / vars are. PyYAML parses such a file happily, which
is why local validation passed it.

Moved to a first step that writes HF_HOME into $GITHUB_ENV via $RUNNER_TEMP
(same value, and `runner` IS allowed inside steps, which is where the two
cache steps already reference it).

Verified with actionlint, which flags the old file at the exact line and passes
the new one — a real workflow validator rather than a YAML parse. Worth wiring
into the lint job so this class cannot recur; not done here to keep this commit
to the regression.

.github/workflows/ci.yml

@@ -49,17 +49,23 @@ jobs:
       # (seen with trl.trainer.grpo_trainer on windows-latest / py3.11).
       PYTHONUTF8: "1"
       PYTHONIOENCODING: "utf-8"
-      # Pin the HF cache to a deterministic path so actions/cache restores it
-      # identically on ubuntu / windows / macos — the default
-      # ~/.cache/huggingface sits under a different user on each runner. Kept
-      # OUTSIDE the checkout on purpose: this lands ~0.5 GB on disk, and inside
-      # the repo it would be walked by the tests that scan cwd for containment.
-      HF_HOME: ${{ runner.temp }}/hf-cache
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
+      - name: Pin the HF cache to a deterministic path
+        # Set here and not in the job-level `env:` block: the `runner` context
+        # is not available there (only github / needs / strategy / matrix /
+        # vars / secrets / inputs are), and referencing it makes GitHub reject
+        # the whole workflow file before any job starts.
+        #
+        # Why a fixed path at all: the default ~/.cache/huggingface sits under
+        # a different user on each runner OS, so actions/cache cannot restore
+        # it by one key. Kept OUTSIDE the checkout because it lands ~0.5 GB,
+        # which the tests that walk cwd for containment would otherwise scan.
+        shell: bash
+        run: echo "HF_HOME=$RUNNER_TEMP/hf-cache" >> "$GITHUB_ENV"
       - name: Restore HF Hub cache
         id: hf-cache
         uses: actions/cache/restore@v4