use github token URL when fetching oauth token

[techknowlogick]

There was a regression in #16544 whereby the default token url
for github was changed to the gitea one.

[codecov-commenter]

Codecov Report

Merging #16709 (5fb9197) into main (ea07726) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main   #16709      +/-   ##
==========================================
+ Coverage   45.38%   45.41%   +0.03%     
==========================================
  Files         758      758              
  Lines       85306    85306              
==========================================
+ Hits        38713    38741      +28     
+ Misses      40313    40286      -27     
+ Partials     6280     6279       -1     

Impacted Files	Coverage Δ
services/auth/source/oauth2/providers_custom.go	63.23% <100.00%> (ø)
modules/queue/unique_queue_disk_channel.go	47.26% <0.00%> (-1.37%)	⬇️
services/pull/pull.go	41.78% <0.00%> (ø)
modules/git/blame.go	67.14% <0.00%> (+1.42%)	⬆️
modules/log/event.go	62.50% <0.00%> (+1.85%)	⬆️
modules/queue/workerpool.go	55.72% <0.00%> (+2.67%)	⬆️
modules/cron/tasks_basic.go	88.34% <0.00%> (+2.91%)	⬆️
models/gpg_key_common.go	64.51% <0.00%> (+4.83%)	⬆️
services/mirror/mirror.go	17.30% <0.00%> (+7.69%)	⬆️
models/repo_mirror.go	69.56% <0.00%> (+10.86%)	⬆️
... and 1 more

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