Install the shfmt tool for GHA

The `shfmt` tool does not ship on the GitHub Actions runners so we must install
it manually.

.github/workflows/build.yml

@@ -15,25 +15,44 @@ jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
+      - uses: cisagov/setup-env-github-action@develop
       - uses: actions/checkout@v2
       - id: setup-python
         uses: actions/setup-python@v2
         with:
           python-version: 3.9
+      # GO_VERSION and GOCACHE are used by the cache task, so the Go
+      # installation must happen before that.
+      - uses: actions/setup-go@v2
+        with:
+          go-version: '1.16'
+      - name: Store installed Go version
+        run: |
+          echo "GO_VERSION="\
+          "$(go version | sed 's/^go version go\([0-9.]\+\) .*/\1/')" \
+          >> $GITHUB_ENV
+      - name: Lookup Go cache directory
+        id: go-cache
+        run: |
+          echo "::set-output name=dir::$(go env GOCACHE)"
       - uses: actions/cache@v2
         env:
           BASE_CACHE_KEY: "${{ github.job }}-${{ runner.os }}-\
-            py${{ steps.setup-python.outputs.python-version }}-"
+            py${{ steps.setup-python.outputs.python-version }}-\
+            go${{ env.GO_VERSION }}-"
         with:
           path: |
             ${{ env.PIP_CACHE_DIR }}
             ${{ env.PRE_COMMIT_CACHE_DIR }}
+            ${{ steps.go-cache.outputs.dir }}
           key: "${{ env.BASE_CACHE_KEY }}\
             ${{ hashFiles('**/requirements-test.txt') }}-\
             ${{ hashFiles('**/requirements.txt') }}-\
             ${{ hashFiles('**/.pre-commit-config.yaml') }}"
           restore-keys: |
             ${{ env.BASE_CACHE_KEY }}
+      - name: Install shfmt
+        run: go install mvdan.cc/sh/v3/cmd/shfmt@${SHFMT_VERSION}
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip