locate: fix `update-locatedb` service for `mlocate`

This fixes the `update-locatedb` service when using the `mlocate`
package.

The service as-is does not properly handle flags during update of the
relevant database when configured to use the `mlocate` package.

The man entry for `updatedb` associated with `mlocate` does not say
that it supports environment variables in place of command line flags,
whereas the `findutils` package's updatedb does so.

To support this distinction, we pass the relevant settings as flags to
the `updatedb` program when using the `mlocate` package.

Fixes #29279