Fix ubuntu to 20.04 in github actions

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        platform: [macos-latest, ubuntu-latest, windows-latest]
+        platform: [macos-latest, ubuntu-20.04, windows-latest]
 
     runs-on: ${{ matrix.platform }}
     steps:
@@ -25,7 +25,7 @@ jobs:
       with:
         toolchain: stable
     - name: Install dependencies (ubuntu only)
-      if: matrix.platform == 'ubuntu-latest'
+      if: matrix.platform == 'ubuntu-20.04'
       run: |
         sudo apt-get update
         sudo apt-get install -y libgtk-3-dev webkit2gtk-4.0 libappindicator3-dev librsvg2-dev patchelf