ci: try ubuntu-22.04 to fix error

https://github.com/racket/gui/actions/runs/14157811194/job/39659667248 has an error `/lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.29' not found (required by /usr/racket-8.16.0.4-tmp-install/bin/racket)`. This might be related to the implicit upgrade to `ubuntu-24.04`, so try `ubuntu-22.04`.

Author: sorawee

.github/workflows/ci.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build-test:
     name: Build & Test (Racket ${{ matrix.variant }})
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     container: racket/racket-ci:latest
 
     strategy: