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
It was failing with a `Read-only filesystem` failure due to the systemd
service option `ReadWriteDirectories` not being correctly configured.
Fixes#14132
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
