Allow setting of environment variables when validating an external script

[Llandy3d]

When validating an external script, we currently only allow to select the script and we automatically create an archive (k6 archive) to run it.
This works well with scripts created in Studio but it has an issue with scripts created externally that might depend on environment variables, since the user doesn't have a way to define them.

This issue proposes to have a small configuration available to define environments variables the user wants to pass to the k6 archive command when opening an external script before proceeding to the validation.

Related Issues

• Validate existing test script which refer other files #435 (comment)