Fix test: tests/pytests/functional/modules/cmd/test_script.py::test_windows_script_args_powershell

[twangboy]

What does this PR do?

The test is failing with the following powershell error:

ConvertTo-SecureString : The 'ConvertTo-SecureString' command was found in the module 'Microsoft.PowerShell.Security', but the module could not be loaded. For more information, run 'Import-Module Microsoft.PowerShell.Security'.

According to StackOverflow the workaround is to clear the PSModulePath environment variable. That's what this tries to do

What issues does this PR fix or reference?

Fixes failing test

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes