vendor: update buildkit to v0.15.0-rc1

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/gofrs/flock/build.sh

@@ -0,0 +1,18 @@
+#!/bin/bash -e
+
+# Not supported by flock:
+# - plan9/*
+# - js/wasm
+# - wasp1/wasm
+
+for row in $(go tool dist list -json | jq -r '.[] | @base64'); do
+  _jq() {
+    echo ${row} | base64 --decode | jq -r ${1}
+  }
+
+  GOOS=$(_jq '.GOOS')
+  GOARCH=$(_jq '.GOARCH')
+
+  echo "$GOOS/$GOARCH"
+  GOOS=$GOOS GOARCH=$GOARCH go build
+done