Do not run "build" workflow when there is no upload secret

The only purpose of this workflow is to upload builds, so do not run it
in contexts when upload is impossible.

TN: TC07-049

.github/workflows/build-binaries.yml

@@ -7,6 +7,12 @@ name: Build binaries
 jobs:
   build:
     name: Build and deploy
+
+    # The only purpose of this workflow is to upload builds, so do not run it
+    # in contexts when upload is impossible: only the official repository has
+    # the secrets to upload binaries to Bintray.
+    if: github.repository = "AdaCore/libadalang"
+
     strategy:
       fail-fast: false
       matrix: # Build debug and production