gst-env: Windows: Fix looking for cmd_or_ps.ps1 in the wrong directory

At the point of get_windows_shell() execution, the current directory
equals DEFAULT_BUILDDIR=./build. But cmd_or_ps.ps1 is in SCRIPTDIR=./
(repo root). Point subprocess.check_output() to the correct directory.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-build/-/merge_requests/238>

gst-env.py

@@ -433,7 +433,7 @@ def get_subprocess_env(options, gst_version):
 
 def get_windows_shell():
     command = ['powershell.exe' ,'-noprofile', '-executionpolicy', 'bypass', '-file', 'cmd_or_ps.ps1']
-    result = subprocess.check_output(command)
+    result = subprocess.check_output(command, cwd=SCRIPTDIR)
     return result.decode().strip()
 
 if __name__ == "__main__":