Migrate from BuildJet to runnerhut

What BuildJet does well

• Pioneered the one-line drop-in positioning the whole category now uses
• buildjet/cache@v4 was a clean, interoperable drop-in for actions/cache

Where we differ

• No longer operating — runners stopped on 31 March 2026
• Customers were directed back to GitHub-hosted runners, not to a faster option
• Any remaining buildjet-* labels in your workflows are now hard failures

The migration

jobs:
  build:
-   runs-on: buildjet-4vcpu-ubuntu-2204
+   runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4

And the migration back

We publish this because after BuildJet's shutdown, "what if you disappear" is a fair question. Leaving is the same single line you changed to arrive.

jobs:
  build:
-   runs-on: runnerhut-8vcpu-ubuntu-2404
+   runs-on: ubuntu-latest

Checklist

1. Install the runnerhut GitHub App on the repositories you want to move
2. Change runs-on in one job first and compare timings against the old runner
3. Add runner.arch to cache keys if you are also moving to arm64
4. Roll out to the rest of the repository once the comparison looks right
5. Set a budget cap and concurrency cancellation before going organisation-wide