Add env variable for machine ID path

Add env variable for machine ID path

During package build, in minimal chroots, or other systems which do not already
have an /etc/machine-id we get six test failures. Introduce a
$SYSTEMD_MACHINE_ID_PATH environment variable which can specify a location
other than /etc/machine-id, so that the unit tests are independent from the
environment.

Also adjust test-fs-util to not assume that /etc/machine-id exists. Use
/etc/issue instead which is from base-files.

Closes: #851445
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=62344

Gbp-Pq: Topic debian
Gbp-Pq: Name Add-env-variable-for-machine-ID-path.patch