Update to ubuntu-latest to fix potential build issue

Author: PicoCreator

.github/workflows/gradle-build.yml

@@ -6,7 +6,7 @@ on:
     branches: [ master ]
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v1
     - name: Init submodules