ci: Run WPT jobs on self-hosted runners

[delan]

this patch enables WPT runs to use self-hosted runners, using the changes in servo/ci-runners#90, servo/ci-runners#91, servo/ci-runners#92, and servo/ci-runners#93 (see servo/ci-runners#21 for more details).

fallback to GitHub-hosted runners is included for now, but we can use three self-hosted runners to do what required twenty GitHub-hosted runners in the same amount of time (20 minutes). in the future, we may be able to run WPT faster than that by decreasing --timeout-multiplier or further increasing --processes, if we fix the flakes.

Testing:

• https://github.com/servo/servo/actions/runs/19951033498 (18 22 18min, 0 stable unexpected)
• https://github.com/servo/servo/actions/runs/19952863638 (18 19 19min, 0 stable unexpected)
• https://github.com/servo/servo/actions/runs/19952882829 (16 24 16min, 0 stable unexpected)
• https://github.com/servo/servo/actions/runs/19952885991 (15 20 20min, 0 stable unexpected)
• https://github.com/servo/servo/actions/runs/19953672646 (16 20 18min, 0 stable unexpected)
• https://github.com/servo/servo/actions/runs/19953675013 (18 17 18min, 0 stable unexpected)

Fixes: part of #38141

[delan]

draft because this needs more testing and tuning to avoid introducing flakes.

[delan]

@sagudev this patch (and the ci-runners patches) should be ready for review :)