Add exec subcommand for runner so that we can run the tasks locally(#39)

Most codes are copied from https://gitea.com/gitea/act/src/branch/main/cmd
and do some small changes to make it run again

examples:

```SHELL
./act_runner exec -l
./act_runner exec -j lint
./act_runner exec -j lint -n
```

some example result:

![屏幕截图 2023-03-06 135735](/attachments/547bd05c-ade2-41f7-ba60-c9937fa32d5f)

![屏幕截图 2023-03-06 140643](/attachments/e8f48dba-c7f3-4daa-a163-aa9b36b1dc32)

Signed-off-by: a1012112796 <1012112796@qq.com>

fix #32

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/39
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: a1012112796 <1012112796@qq.com>
Co-committed-by: a1012112796 <1012112796@qq.com>

cmd/cmd.go

@@ -54,9 +54,11 @@ func Execute(ctx context.Context) {
 		Args:  cobra.MaximumNArgs(1),
 		RunE:  runDaemon(ctx, gArgs.EnvFile),
 	}
-	// add all command
 	rootCmd.AddCommand(daemonCmd)
 
+	// ./act_runner exec
+	rootCmd.AddCommand(loadExecCmd(ctx))
+
 	// hide completion command
 	rootCmd.CompletionOptions.HiddenDefaultCmd = true