fix(solver): check for context canceled in ReleaseUnreferenced

[jsternberg]

The existing implementations do not check for the context being canceled
when ReleaseUnreferenced is called which has caused a bug in how the
controller invokes ReleaseUnreferenced to be uncaught. We pass an
already canceled context to the method which causes any implementations
that do check the context to always fail.

[jsternberg]

Waiting to verify that the CI tests fail properly when the context is checked in the existing code and then I'll amend the commit with the fix.

Found this while working on the sqlite cache backend since the sqlite version of ReleaseUnreferenced honored the context cancellation and caused the error to trigger.

#7216

[jsternberg]

Coming back to this and after talking briefly with @tonistiigi I'm just going to submit the fix here and not bother with tests. Testing this behavior adds far too much code and overhead than is gained from the tests themselves. Since the call to ReleaseUnreferenced here logs instead of failing and I don't want to change that, I'd have to add or change a test to detect when the log message was printed or I'd have to add some mock implementations. None of that is very helpful for such a simple change.