Use the depth= argument from Meson 0.52 to limit the clone depth of
subprojects to 1. This should make the CI images a little smaller, and
reduce the bandwidth required to build them (although that’s not so
important because it only happens once every few months).
Similarly, only clone to a depth of 1 in `cache-subprojects.sh`, which
is run once every few months.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
