ci: introduce the VALVE_FARM variable

ci: introduce the VALVE_FARM variable

This mirrors all the other *_FARM variables, and allows developers to
quickly disable all the jobs that would otherwise run on Valve's CI
infrastructure by setting it to the 'offline' value.

To this end, .radv_rules gets split into .radv-collabora-rules and
.radv-valve-rules, since the driver will be testable in two different
test farms. Every radv job is then made to inherit from the right farm.

Signed-off-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19556>