github: Build the pinned Ubuntu jobs on 22.04

This makes the prebuilt toolchains require 22.04.

The GitHub Actions Ubuntu 20.04 images are deprecated and are
being removed soon, see
https://github.com/actions/runner-images/issues/11101.

.github/workflows/build.yml

@@ -68,7 +68,7 @@ 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
@@ -104,7 +104,7 @@ 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