When I shared srcdir as read-only in lima-vm,
`make install` causes following error:

```
Update rbs to 33813a60752624d58dfe5ae770b39bfaf29fbaf1
error: cannot open .git/FETCH_HEAD: Read-only file system
```

I cannot find any ignore option for `git checkout --detach`
when already checked out. So I add `if`.