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/nix.yml

@@ -20,7 +20,7 @@ jobs:
       matrix:
         system:
           - os: x86 Linux
-            runner: buildjet-16vcpu-ubuntu-2204
+            runner: github-16vcpu-ubuntu-2204
             install_nix: true
           - os: arm Mac
             runner: [macOS, ARM64, test]