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 https://github.com/rustic-beans/dagger-for-github/blob/02882cc2d9faf40e481120cfa4d754a5198c67f2/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.go

@@ -31,6 +31,7 @@ type Runner struct {
 	FetchTimeout    time.Duration     `yaml:"fetch_timeout"`    // FetchTimeout specifies the timeout duration for fetching resources.
 	FetchInterval   time.Duration     `yaml:"fetch_interval"`   // FetchInterval specifies the interval duration for fetching resources.
 	Labels          []string          `yaml:"labels"`           // Labels specify the labels of the runner. Labels are declared on each startup
+	GithubMirror    string            `yaml:"github_mirror"`    // GithubMirror defines what mirrors should be used when using github
 }
 
 // Cache represents the configuration for caching.