sysv-generator-test: do not query $builddir

This variable is not set by meson, so let's not try to use it.

We could use some more elaborate scheme (e.g. based on $MESON_BUILD_ROOT and
$MESON_SUBDIR) to find the path to systemd-sysv-generator, but it seems
that plain ./systemd-sysv-generator works just as well and has the advantage
that it's easy to invoke the test by hand (as long as one cd's to the
meson build dir).