Pin linux-build-examples Ubuntu version

I noticed a discrepancy on external PR GitHub Actions runs vs. upstream main branch GitHub
Actions runs: The main branch CI runs on Ubuntu 22.04, while external PRs run on Ubuntu 24.04.
This causes CI failures due to version mismatches in the distribution package repository.

External PR CI run setup:
https://github.com/aherrmann/buck2/actions/runs/12751410326/job/35538421552#step:1:4
Main branch CI run setup:
https://github.com/facebook/buck2/actions/runs/12749831677/job/35533176968#step:1:4

External PR CI failure:
https://github.com/aherrmann/buck2/actions/runs/12751410326/job/35538421552#step:3:491
Main branch CI success:
https://github.com/facebook/buck2/actions/runs/12749831677/job/35533176968#step:3:461

Author: aherrmann

.github/workflows/build-and-test.yml

@@ -52,7 +52,7 @@ jobs:
     - uses: ./.github/actions/setup_reindeer
     - uses: ./.github/actions/build_bootstrap
   linux-build-examples:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
     - uses: actions/[email protected]
     - uses: ./.github/actions/setup_linux_env