Update gomod and vendor

vendor/github.com/moby/buildkit/util/system/path_unix.go

@@ -0,0 +1,14 @@
+// +build !windows
+
+package system
+
+// DefaultPathEnv is unix style list of directories to search for
+// executables. Each directory is separated from the next by a colon
+// ':' character .
+const DefaultPathEnv = "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+
+// CheckSystemDriveAndRemoveDriveLetter verifies that a path, if it includes a drive letter,
+// is the system drive. This is a no-op on Linux.
+func CheckSystemDriveAndRemoveDriveLetter(path string) (string, error) {
+	return path, nil
+}