builder: have TestBuildDockerignoringBadExclusion pass with buildkit

Signed-off-by: Tibor Vass <tibor@docker.com>

integration-cli/docker_cli_build_test.go

@@ -2534,7 +2534,7 @@ func (s *DockerSuite) TestBuildDockerignoringBadExclusion(c *check.C) {
 		build.WithFile(".dockerignore", "!\n"),
 	)).Assert(c, icmd.Expected{
 		ExitCode: 1,
-		Err:      "error checking context: 'illegal exclusion pattern: \"!\"",
+		Err:      `: illegal exclusion pattern: "!"`,
 	})
 }