Update build.

.github/workflows/build.yml

@@ -11,6 +11,8 @@ jobs:
     # This job runs on Linux
     runs-on: ubuntu-latest
     steps:
+    - name: Install GTK packages
+      run: sudo apt-get install build-essential libgtk2.0-dev
     - uses: actions/checkout@master
     - name: Cache choosenim
       id: cache-choosenim