Set ubuntu runner version to 20.04

This is a quick way to have the binary linked against libc6 2.31: otherwise, the
binary won't run on systems using this version

Author: atextor

.github/workflows/pull_request_check.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        os: [windows-latest, ubuntu-latest]
+        os: [windows-latest, ubuntu-20.04]
     steps:
       - name: Checkout repository
         uses: actions/checkout@v3
@@ -45,7 +45,7 @@ jobs:
           disk-root: "C:"
 
       - name: Set Swap Space (Linux)
-        if: matrix.os == 'ubuntu-latest'
+        if: matrix.os == 'ubuntu-20.04'
         uses: pierotofy/set-swap-space@master
         with:
           swap-size-gb: 12
@@ -62,7 +62,7 @@ jobs:
 
       - name: Upload executable jar
         # We only need one OS job to upload the jars
-        if: matrix.os == 'ubuntu-latest'
+        if: matrix.os == 'ubuntu-20.04'
         uses: actions/upload-artifact@v2
         with:
           name: bamm-cli-jar
@@ -79,7 +79,7 @@ jobs:
             tools/bamm-cli/target/bamm.exe
 
       - name: Upload binary (Linux/Mac)
-        if: matrix.os == 'ubuntu-latest' || matrix.os == 'macos-latest'
+        if: matrix.os == 'ubuntu-20.04' || matrix.os == 'macos-latest'
         uses: actions/upload-artifact@v2
         with:
           name: bamm-cli-binary-${{ matrix.os }}