[ci] Checkout with fetch-depth 2

Otherwise we can't fetch later more commits
for regression-tester and danger.

.github/workflows/build.yml

@@ -22,6 +22,8 @@ jobs:
     if: ${{ !contains(github.event.head_commit.message, '[skip ci]') }}
     steps:
     - uses: actions/checkout@v2
+      with:
+        fetch-depth: 2
     - uses: actions/cache@v2
       with:
         path: |