github: Run the non-asserts linux build on 20.04

This should make the produced artifact more widely compatible.

Releases built with release.sh with the dockerfile are currently
based on Ubuntu 18.04 though, and are more shielded from potential
extra libraries that are found during the build.

.github/workflows/build.yml

@@ -46,7 +46,7 @@ jobs:
   linux:
     if: (github.event_name != 'schedule') || (github.repository == 'mstorsjo/llvm-mingw')
     needs: [prepare]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - name: Build