ci: use full SHAs for github action versions

Ref: https://github.com/expressjs/security-wg/issues/2

.github/workflows/legacy.yml

@@ -36,12 +36,12 @@ jobs:
 
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           persist-credentials: false
 
       - name: Setup Node.js ${{ matrix.node-version }}
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@cdca7365b2dadb8aad0a33bc7601856ffabcc48e # v4.3.0
         with:
           node-version: ${{ matrix.node-version }}
 
@@ -63,7 +63,7 @@ jobs:
         run: npm run test-ci
 
       - name: Upload code coverage
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
         with:
           name: coverage-node-${{ matrix.node-version }}-${{ matrix.os }}
           path: ./coverage/lcov.info
@@ -76,14 +76,14 @@ jobs:
       contents: read
       checks: write
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
 
       - name: Install lcov
         shell: bash
         run: sudo apt-get -y install lcov
 
       - name: Collect coverage reports
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@95815c38cf2ff2164869cbab79da8d1f422bc89e # v4.2.1
         with:
           path: ./coverage
           pattern: coverage-node-*
@@ -93,6 +93,6 @@ jobs:
         run: find ./coverage -name lcov.info -exec printf '-a %q\n' {} \; | xargs lcov -o ./lcov.info
 
       - name: Upload coverage report
-        uses: coverallsapp/github-action@v2
+        uses: coverallsapp/github-action@648a8eb78e6d50909eff900e4ec85cab4524a45b # v2.3.6
         with:
           file: ./lcov.info