Merge pull request #72 from thedadams/fix-cred-override-windows

fix: use separate gptscript for cred override on windows

Author: thedadams

tests/fixtures/credential-override-windows.gpt

@@ -0,0 +1,5 @@
+credentials: github.com/gptscript-ai/credential as test.ts.credential_override with TEST_CRED as env
+
+#!/usr/bin/env powershell.exe
+
+echo "$env:TEST_CRED"