Bug 31479: Option to skip t/00-check-atomic-updates.t when building custom packages

Bug 31479: Option to skip t/00-check-atomic-updates.t when building custom packages

This enhancement adds an option to skip the check for leftover atomic
updates when building custom packages. This is particularly useful for
Koha providers or anyone else building Koha packages manually.

In practice, this could be run like: sudo CUSTOM_PACKAGE=1
./debian/build-git-snapshot -r ~/debian -v 21.11.01git -d

This test plan should all take place within the shell.
0. sudo koha-shell kohadev
1. Run prove t/00-check-atomic-updates.t and confirm the test passes
2. Add a fake atomic update to installer/data/mysql/atomicupdate/ . You
can use the example from
https://wiki.koha-community.org/wiki/Database_updates#How_to_write_an_atomicupdate_file
3. Run prove t/00-check-atomic-updates.t and notice the test fails
4. Set the CUSTOM_PACKAGE environment variable so we can test this. You
can either set on the commandline (using export) or in /etc/environment
(remember to run source /etc/environment so the changes are accessed)
5. Run prove t/00-check-atomic-updates.t and the test should now pass.

Sponsored-by: Catalyst IT
Signed-off-by: Mason James <mtj@kohaaloha.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>