solver: advertise committed cache keys after exec - #6955

TestCacheMissStaleCompleteSharedDep reproduces the concurrency-triggered
miss in-process (no docker): a producer builds step->base fresh, a holder
job keeps only the shared base alive, the producer is discarded, then a
consumer builds a fresh step over the same (already-complete) base. On
pristine code the consumer re-executes (execCallCount=1) instead of
reusing the cached result; with the probeResultCache fix it reuses (0).

Verified to discriminate: fails on the pre-fix tree, passes with fafe01622.

Signed-off-by: Felix de Souza <fdesouza@palantir.com>

An edge that completes by executing never adds its committed cache keys
to edgeState.keys: the pre-execution cache query found nothing (that is
why it executed) and Save only attaches the keys to the result. A
consumer subscribing afterwards therefore receives zero keys, finds no
probeable match, latches noCacheMatchPossible and re-executes even
though its own cached record is intact.

Normally such an edge dies with its build; the next build recreates it,
re-queries the now-populated cache and delivers the key normally. But
when a concurrent build keeps the shared edge referenced in
Solver.actives, a later build reuses it already complete, inherits the
empty key list and re-runs steps that should have been cache hits
(moby#4674 family).

Append the committed result keys to edgeState.keys on completion so an
executed edge advertises its keys the same way a cache-hit edge does.
Skipped for cache loads, whose record key is already present, and for
ignore-cache edges, so that a --no-cache dependency does not let
consumers match records through a step that was forced to re-run.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

The regression test's comments described the earlier consumer-side fix
(probeResultCache reading a completed dep's result keys). The fix that
landed instead makes processExecReq append an executed edge's committed
keys to edgeState.keys on completion. Reword the mechanism references
accordingly; scenarios and assertions are unchanged and still
discriminate: the HeldCompleteBaseMisses subtests fail without the
append, and the ignore-cache subtest fails without its IgnoreCache
guard.

Also use t.Context() instead of context.TODO() and drop t.Parallel().

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>