Revert supporting multiple default actions URLs and use github for exec by default (#262)

## ⚠️ BREAKING ⚠️

Follow https://github.com/go-gitea/gitea/pull/25581 and gitea/act#70 .

- Revert "Parse multiple default actions URLs (#200)"
- Revert "fix defaultActionsUrls config for exec (#233)"
- Use `https://github.com` for exec by default.

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/262

go.mod

@@ -89,4 +89,4 @@ require (
 	gopkg.in/yaml.v2 v2.4.0 // indirect
 )
 
-replace github.com/nektos/act => gitea.com/gitea/act v0.246.1-0.20230620073610-515c2c429d6d
+replace github.com/nektos/act => gitea.com/gitea/act v0.246.1