Refactor environment variables to configuration and registration (#90)

Close #21.

Refactor environment variables to configuration file (config.yaml) and registration file (.runner).

The old environment variables are still supported, but warning logs will be printed.

Like:

```text
$ GITEA_DEBUG=true ./act_runner -c config.yaml daemon
INFO[0000] Starting runner daemon
WARN[0000] env GITEA_DEBUG has been ignored because config file is used

$ GITEA_DEBUG=true ./act_runner daemon
INFO[0000] Starting runner daemon
WARN[0000] env GITEA_DEBUG will be deprecated, please use config file instead
```

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/90
Reviewed-by: Lunny Xiao <[email protected]>

cmd/exec.go

@@ -348,7 +348,7 @@ func runExec(ctx context.Context, execArgs *executeArgs) func(cmd *cobra.Command
 		}
 
 		// init a cache server
-		handler, err := artifactcache.NewHandler()
+		handler, err := artifactcache.NewHandler("", "", 0)
 		if err != nil {
 			return err
 		}