Execute two instances of `freebsd-update fetch` at the same time. This should result in the second instances failing with a locked working directory error.
Instance 1
```
# freebsd-update fetch
Looking up update.FreeBSD.org mirrors... 2 mirrors found.
Fetching metadata signature for 13.1-RELEASE from update1.freebsd.org... done.
Fetching metadata index... done.
Inspecting system... done.
Preparing to download files... done.
...
```
Instance 2
```
# freebsd-update fetch
another freebsd-update(8) instance is running in /var/db/freebsd-update
specify a different work directory with -d
```