build-unit-test-docker: Use `meson setup`

Invoking meson with only a build directory argument is deprecated:

```
2 10:14:32 andrew@fedora:~$ meson --version
1.1.0
2 10:17:32 andrew@fedora:~$ meson builddir
...
WARNING: Running the setup command as `meson [options]` instead of `meson setup [options]` is ambiguous and deprecated.
2 10:17:34 andrew@fedora:~$
```

Change-Id: I29c087cce5b4c2bc928a1cd4c6f2c0afa74a447f
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
1 file changed