Merge pull request #41222 from thaJeztah/19.03_bump_buildkit

[19.03] vendor: moby/buildkit v0.6.4-20-g4cb720ef
Upstream-commit: 5e8ab898c70502db08a7bd293b68e2869da447cf
Component: engine

Author: thaJeztah

components/engine/vendor.conf

@@ -26,7 +26,7 @@ github.com/imdario/mergo                            7c29201646fa3de8506f70121347
 golang.org/x/sync                                   e225da77a7e68af35c70ccbf71af2b83e6acac3c
 
 # buildkit
-github.com/moby/buildkit                            dc6afa0f755f6cbb7e85f0df4ff4b87ec280cb32 # v0.6.4-15-gdc6afa0f
+github.com/moby/buildkit                            4cb720ef6483b43020c9037726de47353ac8ad9b # v0.6.4-20-g4cb720ef
 github.com/tonistiigi/fsutil                        6c909ab392c173a4264ae1bfcbc0450b9aac0c7d
 github.com/grpc-ecosystem/grpc-opentracing          8e809c8a86450a29b90dcc9efbf062d0fe6d9746
 github.com/opentracing/opentracing-go               1361b9cd60be79c4c3a7fa9841b3c132e40066a7