CI: cross-platform locking with restricted runtime platforms

[ncoghlan]

The current test suite doesn't cover the case where a runtime layer only targets a subset of platforms, and an attempt is made to lock that runtime layer (and the other layers that depend on it) from a platform whether that runtime layer isn't enabled.

This scenario only became possible when #254 was implemented, as prior to that point, locking had to be performed separately for each target platform.

The test suite updates in #254 covered cross-platform locking for framework and application layers, but it doesn't currently cover runtime layers.

One potential way to approach this would be via #273, as the example mlx stack omits Windows from the runtime layer up.

[ncoghlan]

There may also be documentation and/or enhancement issue here , as this scenario is currently implicitly requiring that the specified Python runtime be available for the platform running the layer lock, even if it isn't specified as one of the target environments for that runtime layer.

That behaviour predates the switch to universal locking, so it may no longer be necessary to eagerly create the runtime environments before generating the layer locks.