[GitHub Actions] consistently use ubuntu-20.04 and checkout v2 (#28538)

The default fetch-depth has changed, and the epochs workflow needs the
history for `./wpt rev-list` to work.

.github/workflows/epochs.yml

@@ -6,10 +6,12 @@ on:
     - cron: 10 */3 * * *
 jobs:
   update:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - name: Checkout
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
     - name: Run epochs_update.sh
       # Use a conditional step instead of a conditional job to work around #20700.
       if: github.repository == 'web-platform-tests/wpt'

.github/workflows/manifest.yml

@@ -8,14 +8,14 @@ on:
       - 'tools/**'
 jobs:
   build-and-tag:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - name: Set up Python
       uses: actions/setup-python@v2
       with:
         python-version: '3.x'
     - name: Checkout
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2
       with:
         fetch-depth: 50
     - name: Install dependencies