GH Action: Fixed --index argument according to validation rules

Signed-off-by: Ole Herman Schumacher Elgesem <ole.elgesem@northern.tech>

Author: olehermanse

.github/workflows/validate_index.yml

@@ -41,4 +41,4 @@ jobs:
 
       # Validate index fields
       - name: Validate fields
-        run: cfbs validate --index cfbs.json
+        run: cfbs validate --index ./cfbs.json