github: Build on Ubuntu 22.04 instead of Ubuntu 20.04

Github actions no longer provide runners with Ubuntu 20.04.

Author: mstorsjo

.github/workflows/build.yml

@@ -66,7 +66,7 @@ jobs:
   linux:
     if: (github.event_name != 'schedule') || (github.repository == 'mstorsjo/llvm-mingw')
     needs: [prepare]
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
       - name: Build
@@ -109,7 +109,7 @@ jobs:
   linux-cross-aarch64:
     if: (github.event_name != 'schedule') || (github.repository == 'mstorsjo/llvm-mingw')
     needs: [linux, prepare]
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
       - name: Build