chore: change test image docker syntax directive

This was causing an error of the likes of
```ailed to read downloaded context: failed to load cache key: Get
"http://build-context-41y85iqxswexjixqpvc8i3hr5": context not found```
This change will get rid of the error until the main kiln repo is fixed.

internal/test/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile-upstream:master-labs
+# syntax=docker/dockerfile:1
 
 FROM golang as go-image
 FROM pivotalcfreleng/kiln:v0.77.0 as kiln