Fix #4009: reorder the registration of routes

[gsnedders]

Per the mux docs, "Routes are tested in the order they were added to the router. If two routes match, the first one wins."

This means that checks.RegisterRoutes() needs to be called after everything else defining a route in /api/checks/ as it otherwise matches every path under /api/checks/.

Fixes #4009; see also #4010. @past said in the latter said:

Oh, that would probably work, too.

Turns out it doesn't and I don't know why.

In way of minimal testing:

gsnedders@gsnedders-milk wpt.fyi % curl -v -Li --data-urlencode 'run_id=10906214822' --data-urlencode 'owner=web-platform-tests' --data-urlencode 'repo=wpt' --data-urlencode 'artifact_name=safari-technology-preview-results-%2A' 'http://localhost:8080/api/checks/github-actions/'
* Host localhost:8080 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0*   Trying [::1]:8080...
* connect to ::1 port 8080 from ::1 port 52803 failed: Connection refused
*   Trying 127.0.0.1:8080...
* Connected to localhost (127.0.0.1) port 8080
> POST /api/checks/github-actions/ HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/8.7.1
> Accept: */*
> Content-Length: 106
> Content-Type: application/x-www-form-urlencoded
> 
} [106 bytes data]
* upload completely sent off: 106 bytes
< HTTP/1.1 500 Internal Server Error
< Content-Type: text/plain; charset=utf-8
< Strict-Transport-Security: max-age=31536000; preload
< X-Content-Type-Options: nosniff
< Date: Tue, 24 Sep 2024 21:24:46 GMT
< Content-Length: 105
< 
{ [105 bytes data]

100   211  100   105  100   106    404    408 --:--:-- --:--:-- --:--:--   814
* Connection #0 to host localhost left intact
HTTP/1.1 500 Internal Server Error
Content-Type: text/plain; charset=utf-8
Strict-Transport-Security: max-age=31536000; preload
X-Content-Type-Options: nosniff
Date: Tue, 24 Sep 2024 21:24:46 GMT
Content-Length: 105

GET https://api.github.com/repos/web-platform-tests/wpt/actions/runs/10906214822: 401 Bad credentials []

[past]

Trying your curl example pointed out the problem with my testing last night: I was testing with a browser tab to make the request, which of course was making a GET call.

That's what I get for coding late in the evening after a day full of meetings :-)