[ci] make CI $PATH equal for push & PR to cache nonhermetic builds

Change of `PATH` will cause change to hash of nonhermetic builds, which
prevents Verilator build cached in master to be reused in pull request
CI.

It appears that gcloud CLI is the one making difference, so fake a PATH
without installing it (as it's additional ~0.5G to download).

Signed-off-by: Gary Guo <gary.guo@lowrisc.org>

.github/actions/prepare-env/action.yml

@@ -109,6 +109,17 @@ runs:
     - uses: google-github-actions/setup-gcloud@v2
       if: github.event_name != 'pull_request'
 
+    # HACK: for pull request we don't really need gcloud (and it's ~0.5G to download), but not including
+    # it in the PATH cause environment variables to change and Verilator build to be uncached.
+    - name: Fake gcloud PATH
+      if: github.event_name == 'pull_request'
+      shell: bash
+      run: |
+        LATEST=$(curl -sSfL https://raw.githubusercontent.com/google-github-actions/setup-cloud-sdk/main/data/versions.json | jq -r .[-1])
+        GCLOUD_PATH="${{ runner.tool_cache }}/gcloud/$LATEST/x64/bin"
+        mkdir -p "$GCLOUD_PATH"
+        echo "$GCLOUD_PATH" >> "$GITHUB_PATH"
+
     - name: Configure ~/.bazelrc
       if: inputs.configure-bazel == 'true'
       run: |