Audacity does not check permissions when the folder path contains only drive letter

**Describe the bug**
When a user sets only drive letter for [Preferences - Directories - Temporary files directory - Location],
Audacity doesn't check the permissions to the directory and saves it successfully.

**To Reproduce**
Steps to reproduce the behaviour:
1. Go to Preferences - Directories - Temporary files directory
2. Set 'C:' as the value for the 'Location' field, Click the Ok button
3. Restart Audacity

**Actual behaviour**
Audacity raises an error while launching.

**Expected behaviour**
Audacity checks directory permissions when a user clicks on the Ok button in the Preferences - - Temporary files directory

**Screenshots**

**Additional information (please complete the following information):**
- OS: Windows 10
- Version Audacity 3.1.0-alpha-20210903

Start with the Preferences > Directories > Temporary files directory setting and trace how the Location value is validated on Windows when OK is clicked. Reproduce the C: case on Windows 10, then verify that permissions are checked before saving and that Audacity does not fail on restart.

Written by the indexing model from the issue text.