Use latest version of actions/checkout in Go release workflow

Previously, a significantly outdated version of the `actions/checkout` GitHub Actions workflow was in use. The modern
version is already in use in the Arduino Lint repository without any problems, so there is no reason to stick with the
old one.

Author: per1234

workflow-templates/dependabot/workflow-template-copies/.github/workflows/publish-go-nightly-task.yml

@@ -15,7 +15,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
         with:
           fetch-depth: 0
 

workflow-templates/publish-go-nightly-task.yml

@@ -15,7 +15,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
         with:
           fetch-depth: 0