snap: use "base: bare"

Currently the gtk-common-themes snap uses base: core18. This is unfortunate if the only desktop snaps on the system are using core20 (or some other future base), since it will force the installation of the old core18 base (58MB). We could avoid this by switching to base: core20, but that imposes a similar penalty on systems that only have core18 snaps installed.

This MR takes another path: switch to the "bare" base (4KB), so there is minimal overhead for both mixes of packages. We select the build environment using the build-base directive. As a bonus, this means we can switch to new build environments without requiring users install the corresponding large bases.