chore(workflows): bump actions/checkout to v2

actions/checkout v2 clones shallow repos by default.
I've set the fetch-depth to 0 to prevent this, because goreleaser requires the
full history available locally in order to generate the release notes
automatically.

.github/workflows/release.yml

@@ -11,7 +11,10 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+
     - uses: actions/setup-go@v1
       with:
         go-version: 1.14