Use regex where we ignore case on windows

[KaiSforza]

Description

I was just about to start testing things, and was running into failed tests on main. Both test_invalid_gitignore and test_invalid_nested_gitignore both threw errors because of the case-sensitivity of "s" in "STRING" with gitignore being c:\\... and the output having C:\\....

On windows the path FoObAR is the same as foobar, so the output of black on a windows machine could output the path to .gitignore with upper or lower-case portions of the path. In this change we use the regex re.IGNORECASE flag to get it to match properly. We do not do this when the gitignore path is not a pathlib.WindowsPath object.

Checklist - did you ...

• Add an entry in CHANGES.md if necessary? - not necessary
• Add / update tests if necessary? - these are the tests 😅
• Add new / update outdated documentation? - don't think any documentation is needed

Updated test output

Running pytest with args: ['-p', 'vscode_pytest', '--rootdir', 'c:\\Users\\kai\\git\\black', 'c:\\Users\\kai\\git\\black\\tests\\test_black.py::TestFileCollection::test_invalid_nested_gitignore', 'c:\\Users\\kai\\git\\black\\tests\\test_black.py::TestFileCollection::test_invalid_gitignore']
============================= test session starts =============================
platform win32 -- Python 3.11.4, pytest-8.0.2, pluggy-1.4.0
rootdir: c:\Users\kai\git\black
configfile: pyproject.toml
plugins: cov-4.1.0, xdist-3.5.0
collected 2 items

tests\test_black.py ..                                                   [100%]

============================== 2 passed in 0.14s ==============================
Finished running tests!

[KaiSforza]

I don't think this change will require a change in CHANGES.md.