Only run build when not in forked repo

Environment Secrets don't get passed to fork workflows,
which breaks those workflows

See https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows#pull-request-events-for-forked-repositories
With the exception of GITHUB_TOKEN, secrets are not passed to the
runner when a workflow is triggered from a forked repository.

.github/workflows/build.yml

@@ -9,6 +9,7 @@ permissions:
 jobs:
   build:
     name: Build binaries
+    if: ${{ github.repository == 'FiloSottile/age' }}
     runs-on: ubuntu-latest
     environment: "Build, sign, release binaries"
     steps: