direnv mangles environment variables on windows

[sprig]

Describe the bug
On windows, direnv (even with an empty .envrc) attempts to upcase environment variables that it shouldn't be touching since they are not modified in the config;

T:\projects\test> direnv allow                                                                               
T:\projects\test> direnv export pwsh                                                                   
direnv: loading /t/projects/test/.envrc
direnv: export +COMMONPROGRAMFILES +COMSPEC +PATH +PROGRAMFILES +SYSTEMDRIVE +SYSTEMROOT +WINDIR -ComSpec -CommonProgramFiles -Path -ProgramFiles -SystemDrive -SystemRoot -windir

Furthetmore, it appears that the expression that direnv produces for powershell is malformed. Note the double single quotes:

Remove-Item -Path 'env:/'SystemRoot'';Remove-Item -Path 'env:/'SystemDrive'';$env:PROGRAMFILES='C:\Program Files';$env:COMMONPROGRAMFILES='C:\Program Files\Common Files';$env:COMSPEC='C:\Windows\system32\cmd.exe';$env:SYSTEMDRIVE='C:';Remove-Item -Path 'env:/'ComSpec'';Remove-Item -Path 'env:/'windir'';Remove-Item -Path 'env:/'CommonProgramFiles'';$env:WINDIR='C:\Windows';
...

And it produces the following errors, as in #1214, either when hooking into the shell or trying to source the output of direnv export pwsh:

Remove-Item: A positional parameter cannot be found that accepts argument 'SystemDrive'.
Remove-Item: A positional parameter cannot be found that accepts argument 'Path'.
Remove-Item: A positional parameter cannot be found that accepts argument 'ProgramFiles'.
Remove-Item: A positional parameter cannot be found that accepts argument 'CommonProgramFiles'.
Remove-Item: A positional parameter cannot be found that accepts argument 'ComSpec'.
Remove-Item: A positional parameter cannot be found that accepts argument 'SystemRoot'.
Remove-Item: A positional parameter cannot be found that accepts argument 'windir'

Isolating just the produced expression for the comspec variable, we get:

❯ cat .\comspec
Remove-Item -Path 'env:/'ComSpec''

T:\projects\test
❯ cat .\comspec|Invoke-Expression
Remove-Item: A positional parameter cannot be found that accepts argument 'ComSpec'.

To Reproduce
Create en empty directory with an empty .envrc. direnv allow on it and run direnv export pwsh (same happens if the hook is added to the profile startup script).

Expected behavior
I would expect environment variables that are not modified via .envrc (or .env if .envrc loads that) to not be touched.

Environment

• OS: Windows 11
• Shell: powershell 7.4.2
• Direnv version: 2.34.0
• Bash version 5.2.15(1)

Additional context
direnv.toml has bash set to C:\Program Files\Git\usr\bin\bash.exe, which is installed by git to avoid using the wsl bash and the error from #1214.

I haven't looked in the code yet but I suspect this is somehow related to the interaction with mingw64 bash. likely a diff between the environment as shown by env in bash (that has environment variables upcased) and the environment as seen by powershell produces this effect. No clue what causes the malformed expressions themselves (i.e. that produce an error).