CI: Exercise a DESTDIR installation

This is what packagers will use.

Signed-off-by: Simon McVittie <smcv@debian.org>
3 jobs for meson-post-install in 10 minutes and 20 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #10769500
build_autotools

00:06:41

passed #11818067
build_stable

00:03:38

passed #10769499
build_stable

00:03:54