Fix a bug in jobs w/o sccache setup (#27)

* Only forward sccache variables when they're valid

* Use arrays instead of strings

* Use github substitution instead of bash

Author: mkruskal-google

internal/docker-run/action.yml

@@ -81,12 +81,21 @@ runs:
       if: ${{ !inputs.docker-cache }}
       run: time docker pull -q ${{ inputs.image }}
 
-    - name: Run docker
+    - name: Forward sccache arguments
       shell: bash
+      id: sccache
+      if: ${{ env.SCCACHE_GCS_KEY_PATH != '' }}
       run: >
-        time docker run ${{ inputs.run-flags}} -v${{ github.workspace }}:/workspace
+        echo "args="
         -e SCCACHE_GCS_RW_MODE=$SCCACHE_GCS_RW_MODE
         -e SCCACHE_GCS_BUCKET=$SCCACHE_GCS_BUCKET
         -e SCCACHE_GCS_KEY_PREFIX=$SCCACHE_GCS_KEY_PREFIX
-        -e SCCACHE_GCS_KEY_PATH=/workspace/$(basename $SCCACHE_GCS_KEY_PATH || echo "unknown")
+        -e SCCACHE_GCS_KEY_PATH=/workspace/$(basename $SCCACHE_GCS_KEY_PATH)
+        "" >> $GITHUB_OUTPUT
+
+    - name: Run docker
+      shell: bash
+      run: >
+        time docker run ${{ steps.sccache.outputs.args }}
+        ${{ inputs.run-flags}} -v${{ github.workspace }}:/workspace
         ${{ inputs.image }} ${{ inputs.command }}