Improve flaky test architecture integration with Debian and installed-tests
See suggestions in !579 (comment 403871) and #1515 (comment 398596):
In Debian I've patched [some tests] to be skipped unless a Debian-specific environment variable
DEB_ALLOW_FLAKY_TESTS
is set. We now skip those tests at build-time and for the normal run of the installed-tests in autopkgtest (Debian's equivalent of GNOME installed-tests). We also do a secondary run of the installed-tests, running only the subset that is conditionally skipped, with the environment variable set to enable everything; this secondary run is marked as "flaky", so that its failures are recorded for information but not treated as errors.
A typical example is https://salsa.debian.org/gnome-team/glib/blob/debian/2.62.0-1/debian/patches/debian/gmenumodel-test-Mark-as-flaky.patch.
Justification for using environment variables: essentially the same reasoning as #1967 (closed).