contenthash: data race

Multiple calls to GetCacheContext may occur from multiple threads;
however they should only occur when dealing with an ImmutableRef.
For this case we still need to perform a lock to prevent data race
warnings; however in reality the linkMap will be unused.

Signed-off-by: Alex Couture-Beil <alex@earthly.dev>

cache/contenthash/checksum.go

@@ -110,7 +110,9 @@ func (cm *cacheManager) GetCacheContext(ctx context.Context, md cache.RefMetadat
 	cm.lruMu.Unlock()
 	if ok {
 		cm.locker.Unlock(md.ID())
+		v.(*cacheContext).mu.Lock() // locking is required because multiple ImmutableRefs can reach this code; however none of them use the linkMap.
 		v.(*cacheContext).linkMap = map[string][][]byte{}
+		v.(*cacheContext).mu.Unlock()
 		return v.(*cacheContext), nil
 	}
 	cc, err := newCacheContext(md)