.github: change ubuntu-latest to ubuntu-22.04 (#8)

It's best practice to pin the OS version, and the Exercism docs
recommend this [1].

[1] https://github.com/exercism/docs/blob/63e7a75267c2b62a89a9cce518b0e25dfeb8c57e/building/github/gha-best-practices.md?plain=1#L108-L127

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ on:
 jobs:
   build:
     name: Tests
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout code
         uses: actions/checkout@8e5e7e5ab8b370d6c329ec480221332ada57f0ab # 2.3.5