Fix: uploading artifacts of failed actions to remote cache stopped working.

To reproduce: run a failing test with --experimental_remote_spawn_cache or with --spawn_strategy=remote and no executor. Expected: test log is uploaded.

Desired behavior:
- regardless of whether a spawn is cacheable or not, its artifacts should be uploaded to the remote cache.
- the spawn result should only be set if the spawn is cacheable *and* the action succeeded.
- when executing remotely, the do_not_cache field should be set for non-cacheable spawns, and the remote execution engine should respect it.

This CL contains multiple fixes to ensure the above behaviors, and adds a few tests, both end to end and unit tests. Important behavior change: it is no longer assumed that non-cacheable spawns should use a NO_CACHE SpawnCache! The appropriate test case was removed. Instead, an assumption was added that all implementations of SpawnCache should respect the Spawns.mayBeCached(spawn) property. Currently, only NO_CACHE and RemoteSpawnCache exist, and they (now) support it.

TESTED=remote build execution backend.
WANT_LGTM: philwo,buchgr
RELNOTES: None
PiperOrigin-RevId: 178617937