ci: Limit depth of subprojects when cloning

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>

subprojects/libffi.wrap

@@ -2,3 +2,4 @@
 directory=libffi
 url=https://gitlab.freedesktop.org/gstreamer/meson-ports/libffi.git
 revision=meson
+depth=1