deps: update buildkit, vendor changes

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

vendor/golang.org/x/sys/plan9/pwd_go15_plan9.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.5
-// +build go1.5
 
 package plan9
 

vendor/golang.org/x/sys/plan9/pwd_plan9.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !go1.5
-// +build !go1.5
 
 package plan9
 

vendor/golang.org/x/sys/plan9/race.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build plan9 && race
-// +build plan9,race
 
 package plan9
 

vendor/golang.org/x/sys/plan9/race0.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build plan9 && !race
-// +build plan9,!race
 
 package plan9
 

vendor/golang.org/x/sys/plan9/str.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build plan9
-// +build plan9
 
 package plan9
 

vendor/golang.org/x/sys/plan9/syscall.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build plan9
-// +build plan9
 
 // Package plan9 contains an interface to the low-level operating system
 // primitives. OS details vary depending on the underlying system, and

vendor/golang.org/x/sys/plan9/zsyscall_plan9_386.go

@@ -2,7 +2,6 @@
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 //go:build plan9 && 386
-// +build plan9,386
 
 package plan9
 

vendor/golang.org/x/sys/plan9/zsyscall_plan9_amd64.go

@@ -2,7 +2,6 @@
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 //go:build plan9 && amd64
-// +build plan9,amd64
 
 package plan9
 

vendor/golang.org/x/sys/plan9/zsyscall_plan9_arm.go

@@ -2,7 +2,6 @@
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 //go:build plan9 && arm
-// +build plan9,arm
 
 package plan9