os-release: Introduce experiment RELEASE_TYPE

os-release: Introduce experiment RELEASE_TYPE

This is for experimental builds of the OS made to test some specific WIP
feature.

For example, let's say the distro in question is Asahi Linux and Apple
just released the M3 SoC. The Asahi developers will start porting to the
M3, and will quickly generate builds of Asahi Linux that can technically
boot but aren't ready for any kind of daily use. These images are marked
as experimental, and can be shared among the developers. If a user
somehow stumbles upon one of these images and tries to install it,
they'll be warned that they're about to install an experimental Apple M3
port of Asahi Linux. Eventually, once the Asahi developers think that
their M3 port is ready for a wider audience, they can merge it into the
mainline Asahi repos, where it will be distributed through the usual
nightly CI builds (where RELEASE_TYPE=pre-release; M3 support is no
longer experimental).