this patch extends servo#39900, making lint jobs run on self-hosted runners
if available.

Testing:
- before (15min)
<https://github.com/servo/servo/actions/runs/18532931828/job/52820332815>
- after (&lt;3min)
<https://github.com/servo/servo/actions/runs/18839433347/job/53748113347>

Fixes: part of servo#38141

Signed-off-by: Delan Azabani <dazabani@igalia.com>

currently our self-hosted runner system falls back to github-hosted
runners if there’s no available capacity at the exact moment of the
select runner request. this is suboptimal, because if the job would take
5x as long on github-hosted runners, then you could wait up to 80% of
that time for a self-hosted runner and still win.

this patch implements a new global queue service that allows self-hosted
runner jobs to wait for available capacity. the service will run on [one
server](https://ci0.servo.org) for now, as a single queue that
dispatches to all available servers, like any efficient supermarket.
queueing a job works like this:

1. **POST
/profile/<profile_key>/enqueue?<unique_id>&<qualified_repo>&<run_id>**
(tokenful)
or **POST
/enqueue?<unique_id>&<qualified_repo>&<run_id>**
(tokenless) to enqueue a job.
- both endpoints return a random token that is used to authenticate the
client in the next step.
- the tokenless endpoint validates that the request came from an
authorised job, [using an
artifact](servo/servo#39900).
- the request is rejected if no servers are configured to target
non-zero runners for the requested profile, because we may never be able
to satisfy it.
- there are no limits to queue depth (at least not yet), but clients
probably have better knowledge of the nature of their job anyway, and in
theory, they could use that knowledge to decide how long to wait (see
below).

2. **POST /take/<unique_id>?<token>** to try to take the runner
for the enqueued job. once capacity is available, this endpoint is
effectively proxied to POST /profile/<profile_key>/take on one of the
underlying servers.
- if the client failed to provide the correct token from the previous
step, the response is HTTP 403.
- if the unique id is unknown, because it expired or the queue service
restarted, the response is HTTP 404.
- if there’s no capacity yet, the response is HTTP 503. repeat after
waiting for ‘Retry-After’ seconds.
- if taking the runner was successful, the response is HTTP 200, with
the runner details as JSON.
- if taking the runner was somehow unsuccessful (bug), the response is
HTTP 200, with `null` as JSON. this sucks, to be honest, but it was also
true for the underlying monitor API.
     - when we fix this, we should be careful about curl --retry.
- clients are free to abandon a queued job without actually taking it,
by doing nothing for 30 seconds. for now, the runner-select action
client abandons a queued job if it has been waiting for one hour.

i’ve added a “self-test” workflow that can be manually dispatched to
test the new flow (e.g. [ok
1](https://github.com/delan/servo-ci-runners/actions/runs/19192407233/job/54868629052#step:3:138),
[ok
2](https://github.com/delan/servo-ci-runners/actions/runs/19192417407/job/54868653437#step:3:138),
[ok
3](https://github.com/delan/servo-ci-runners/actions/runs/19192424667/job/54868671844#step:3:138),
[unsatisfiable](https://github.com/delan/servo-ci-runners/actions/runs/19192441772/job/54868712126#step:3:138),
[unauthorised](https://github.com/delan/servo-ci-runners/actions/runs/19192458274/job/54868749313#step:3:138)).
you can also play around with this locally by spinning up a monitor and
a queue on your own machine, then sending the requests by hand (so three
separate terminals):

- `$ cargo build && sudo IMAGE_DEPS_DIR=$(nix eval --raw .\#image-deps)
LIB_MONITOR_DIR=. $CARGO_TARGET_DIR/debug/monitor`
- `$ cargo build && sudo IMAGE_DEPS_DIR=$(nix eval --raw .\#image-deps)
LIB_MONITOR_DIR=. $CARGO_TARGET_DIR/debug/queue`
- `$ unique_id=$RANDOM; curl --fail-with-body -sSX POST --retry-max-time
3600 --retry 3600 --retry-delay 1
'http://192.168.100.1:8002/take/'"$unique_id"'?token='"$(curl
--fail-with-body -sSX POST --oauth2-bearer "$SERVO_CI_MONITOR_API_TOKEN"
'http://192.168.100.1:8002/profile/servo-windows10/enqueue?unique_id='"$unique_id"'&qualified_repo=delan/servo&run_id=123')"`