[Dev] ~50 library tests fail with the wrong drive on Windows

[citelao]

The tests are failing when run on any Windows machine that uses the default system drive of C:/ (they hard-code D:/).

Problem

# Set up beets for contributing (install poetry, etc)
# ...

poe test
# => 50+ failures, like:
# FAILED test/test_library.py::DestinationFunctionTest::test_upper_case_literal - AssertionError: assert b'C:\\base\\FOO' == b'D:\\base\\FOO'

This looks to be a regression introduced with #5224. The fix is to use the %SystemDrive% environment variable, not a hard-coded D:

Setup

• OS: Windows 11 24H2
• Python version: 3.11
• beets version: commit e439c04
• Turning off plugins made problem go away (yes/no): (n/a)