github/workflows: downgrade golang to 1.12

We currently declare compatibility with golang 1.12 so we should also
use it in our CI. Once we agree that we would benefit from some features
in golang 1.13 or 1.14 we can bump it back.

.github/workflows/tests.yml

@@ -16,10 +16,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-      - name: Set up Go 1.13
+      - name: Set up Go 1.12
         uses: actions/setup-go@v1
         with:
-          go-version: 1.13
+          go-version: 1.12
         id: go
 
       - name: Check out code into the Go module directory