fix(cache): use correct mmap-go constant for read-write mapping

mmap.MapRegion expects the library's own constants (mmap.RDWR), not raw
unix PROT_* flags. Passing unix.PROT_READ|unix.PROT_WRITE (=3) happened
to work due to numerical coincidence, but is fragile.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Babis Chalios <babis.chalios@e2b.dev>

Author: bchalios

packages/orchestrator/pkg/sandbox/block/cache.go

@@ -85,7 +85,7 @@ func NewCache(size, blockSize int64, filePath string, dirtyFile bool) (*Cache, e
 		return nil, fmt.Errorf("size too big: %d > %d", size, math.MaxInt)
 	}
 
-	mm, err := mmap.MapRegion(f, int(size), unix.PROT_READ|unix.PROT_WRITE, 0, 0)
+	mm, err := mmap.MapRegion(f, int(size), mmap.RDWR, 0, 0)
 	if err != nil {
 		return nil, fmt.Errorf("error mapping file: %w", err)
 	}