Local exporter hangs if attestation content is larger than 2^22 (4194304) bytes

[jedevc]

If the attestation content exceeds the specified size (4194304 bytes = 4 MB), then the GRPC layer rejects the malformed packet during file transfer with fsutil.

#14 exporting to client directory
#14 copying files
#14 copying files 9.59MB 5.1s
#14 copying files 56.60MB 10.2s
#14 copying files 61.14MB 15.2s
#14 copying files 64.32MB 20.3s
#14 copying files 68.11MB 25.3s
#14 copying files 70.21MB 30.4s
#14 copying files 72.55MB 35.4s
#14 copying files 135.74MB 40.5s
#14 copying files 184.37MB 45.6s
#14 copying files 188.00MB 50.6s
#14 copying files 213.86MB 55.6s
#14 copying files 217.27MB 60.6s
#14 copying files 225.96MB 65.6s
#14 copying files 229.45MB 70.7s
#14 copying files 276.34MB 75.7s
#14 copying files 346.11MB 80.8s
#14 copying files 361.03MB 85.9s
#14 copying files 363.68MB 90.9s
#14 copying files 364.19MB 96.0s
#14 copying files 367.78MB 101.1s
#14 copying files 376.29MB 106.1s
#14 copying files 377.83MB 111.2s
#14 copying files 379.99MB 116.6s
#14 copying files 385.10MB 121.7s
#14 copying files 389.25MB 126.7s
#14 copying files 395.87MB 131.8s
#14 copying files 396.40MB 136.8s
#14 copying files 399.14MB 141.9s
#14 copying files 442.43MB 146.9s
#14 copying files 444.43MB 150.8s done
#14 ERROR: rpc error: code = ResourceExhausted desc = grpc: received message larger than max (5338399 vs. 4194304)
------
 > exporting to client directory:
------
ERROR: failed to solve: ResourceExhausted: rpc error: code = ResourceExhausted desc = grpc: received message larger than max (5338399 vs. 4194304)

To prevent this, we need to patch fsutil, e.g. with tonistiigi/fsutil#144. The StaticFS implementation in buildkit returns bytes.Reader objects, which implement the io.WriterTo interface, which circumvents the packet size limits in fsutil.

[jedevc]

Fixed by #3461.