CI: Add an explicit timeout

Not sure why Ubuntu tests are occasionally taking a very long time.

Author: vHanda

.github/workflows/dart.yml

@@ -45,6 +45,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, macOS-latest] # FIXME: Add windows-latest, need the git executable!
         sdk: [stable]
+    timeout-minutes: 15
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/[email protected]