ci: Switch from BuildJet to GitHub runners (#35826)

In response to an ongoing BuildJet outage, consider migrating CI to
GitHub hosted runners.

Also includes revert of (causing flaky tests):
- https://github.com/zed-industries/zed/pull/35741

Downsides:
- Cost (2x)
- Force migration to Ubuntu 22.04 from 20.04 will bump our glibc minimum
from 2.31 to 2.35. Which would break RHEL 9.x (glibc 2.34), Ubuntu 20.04
(EOL) and derivatives.

Release Notes:

- N/A

.github/workflows/randomized_tests.yml

@@ -20,7 +20,7 @@ jobs:
     name: Run randomized tests
     if: github.repository_owner == 'zed-industries'
     runs-on:
-      - buildjet-16vcpu-ubuntu-2204
+      - github-16vcpu-ubuntu-2204
     steps:
       - name: Install Node
         uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4