[github-actions] only test egui project

Work-around for weird build issue in
https://github.com/emilk/emigui/runs/927167824?check_suite_focus=true

.github/workflows/rust.yml

@@ -30,6 +30,7 @@ jobs:
       - uses: actions-rs/cargo@v1
         with:
           command: test
+          arg: -p egui # Only test egui due to weird build issues with the others
 
   fmt:
     name: Rustfmt