Ignore mkosi.builddir. In the future we can also add other patterns
if necessary.
run-intergration-tests.sh is updated to use the new script, and modified
to work from arbitrary directory.
Follow-up for #7494.
Automatic rebuilding is removed: it doesn't play well with ninja, because
ninja always writes logs, and even if nothing needs to be built, it will
make the log file owned by root. So let's just remove this, and say that
the user must always do the build first.
I'm also keeping make for the tests, because ninja doesn't play well with
sudo.
Since the build directory is arbitrary, it needs to be specified, e.g.
sudo make BUILD_DIR=/home/zbyszek/src/systemd/build1 -C test/TEST-01-BASIC/
