chore: don't build images for dependabot PRs

dependabot doesn't have permission to push to the image registry, so any
attempts cause CI to fail.

.github/workflows/build.yaml

@@ -27,6 +27,7 @@ jobs:
         name: dist.tar
         path: dist.tar
   buildimage:
+    if: ${{ !startsWith(github.head_ref, 'dependabot/') }}
     strategy:
       matrix:
         binary:
@@ -35,8 +36,6 @@ jobs:
     needs: build
     runs-on: ubuntu-latest
     steps:
-    - name: Debug head ref
-      run: echo ${{ github.head_ref }}
     - name: Checkout
       uses: actions/checkout@v3
     - name: Download binaries tar file