solver: correct cache nil return value in Exec

[jedevc]

Follow up to #3043.

We introduce a new boolean execDone to track whether we've successfully completed an Exec call in the past, since nil is a possible return value from Exec.

We have some similar logic for cacheDone set by CacheMap, hence the name of the new bool, but I wonder if we might want to handle nil return values for that as well? Not sure if that would ever trigger, and also not super familiar with the logic here 👀

buildkit/solver/jobs.go

Lines 747 to 752 in c58f128

	if s.cacheRes != nil && s.cacheDone || index < len(s.cacheRes) {
	return s.cacheRes, nil
	}
	if s.cacheErr != nil {
	return nil, s.cacheErr
	}