Update manual-build.yml

Author: tsterbak

.github/workflows/manual-build.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: ['ubuntu-latest', 'macos-latest', 'windows-latest']
+        os: ['ubuntu-20.04', 'ubuntu-latest', 'macos-latest', 'windows-latest']
 
     steps:
     - uses: actions/checkout@v2