reintroduce condition for signing the tarball

[ci skip]

.github/workflows/build.yml

@@ -44,6 +44,7 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }} # Needed to get PR information, if any
           SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}
       - name: Sign source tarball with key 615D449FE6E6A235
+        if: startsWith(github.ref, 'refs/tags/')
         run: |
           git archive --prefix="cryptomator-${{ github.ref_name }}/" -o "cryptomator-${{ github.ref_name }}.tar.gz" ${{ github.ref }}
           echo "${GPG_PRIVATE_KEY}" | gpg --batch --quiet --import