CI: Only main branch pushes bst artifacts to Endless cache server

Pushing artifacts takes time. Pushing developing bst artifacts to cache
server bothers other development, too. So, make it only main branch push
bst artifacts to Endless cache server.

Author: starnight

.github/workflows/build.yml

@@ -63,6 +63,11 @@ jobs:
           echo "${ENDLESSCLIENT_CA_CHAIN}" >> client.crt
           echo "${ENDLESSCLIENT_KEY}" > client.key
           chmod 600 client.key
+          if [ "${{ github.ref_name }}" == "main" ]; then
+            PUSH_ARTIFACTS="true"
+          else
+            PUSH_ARTIFACTS="false"
+          fi
           mkdir -p ~/.config
           cat > ~/.config/buildstream.conf << EOF
           # BuildStream user configuration
@@ -89,7 +94,7 @@ jobs:
           artifacts:
             servers:
             - url: https://bstcache.endlessos.org
-              push: true
+              push: ${PUSH_ARTIFACTS}
               auth:
                 client-cert: $(pwd)/client.crt
                 client-key: $(pwd)/client.key