Add checks: write permission to standardrb workflow

Dependabot PRs run with a restricted GITHUB_TOKEN that lacks checks:write
by default, causing the standard-ruby-action to fail with a 403 when it
tries to create a Check Run. Explicitly granting checks: write (and
contents: read for checkout) fixes this.

Author: dduugg

.github/workflows/standardrb.yaml

@@ -5,6 +5,9 @@ on: [push]
 jobs:
   build:
     runs-on: ubuntu-latest
+    permissions:
+      checks: write
+      contents: read
     steps:
     - uses: actions/checkout@v1
     - name: StandardRB Linter