feat: support github mirror (#716)

ref: https://github.com/go-gitea/gitea/issues/34858

when github_mirror='https://ghfast.top/https://github.com'

it will clone from the github mirror

However, there is an exception: because the cache is hashed using the string, if the same cache has been used before, it will still be pulled from github, only for the newly deployed act_ruuner

In the following two scenarios, it will solve the problem encountered:

1. some github mirror is  https://xxx.com/https://github.com/actions/checkout@v4, it will report error `Expected format {org}/{repo}[/path]@ref. Actual ‘https://xxx.com/https://github.com/actions/checkout@v4’ Input string was not in a correct format`
2. If I use an action that has a dependency on another action, even if I configure the url of the action I want to use, the action that the action introduces will still pull from github.
for example 02882cc2d9/action.yml (L127-L132)

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/716
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Akkuman <akkumans@qq.com>
Co-committed-by: Akkuman <akkumans@qq.com>

internal/pkg/config/config.example.yaml

@@ -32,6 +32,11 @@ runner:
   fetch_timeout: 5s
   # The interval for fetching the job from the Gitea instance.
   fetch_interval: 2s
+  # The github_mirror of a runner is used to specify the mirror address of the github that pulls the action repository.
+  # It works when something like `uses: actions/checkout@v4` is used and DEFAULT_ACTIONS_URL is set to github,
+  # and github_mirror is not empty. In this case,
+  # it replaces https://github.com with the value here, which is useful for some special network environments.
+  github_mirror: ''
   # The labels of a runner are used to determine which jobs the runner can run, and how to run them.
   # Like: "macos-arm64:host" or "ubuntu-latest:docker://docker.gitea.com/runner-images:ubuntu-latest"
   # Find more images provided by Gitea at https://gitea.com/docker.gitea.com/runner-images .