[Bug]: special locations ignore XDG environment variables

[umlaeute]

Detailed steps on how to reproduce the bug

• configure JUCE to build the extras
• build JUCE with extras
• export the XDG_CONFIG_HOME envvar, setting it to /tmp/
• run the SystemInfoDemo:

  XDG_CONFIG_HOME=/tmp/ ./SystemInfoDemo_artefacts/SystemInfoDemo

notice that the SystemInfoDemo window says:

> User application data folder:   /home/user/.config

### What is the expected behaviour?

i expect it to honour the `XDG_CONFIG_HOME` *envvar, so that SystemInfoDemo says:

> User application data folder:   /tmp



note, that adding a line `XDG_CONFIG_HOME=/tmp`  to the `~/.config/user-dirs.dirs` file works.

however, when i read the [specifications](https://specifications.freedesktop.org/basedir-spec/latest) i only see that the *environment variables* must be honoured.
i guess that falling back to values in `~/.config/user-dirs.dirs` is fine, as long as the environment variables take precedence.

it should be simple enough to first check the envvar whether it exists, before parsing the `user-dirs.dirs` file for a fallback.

### Operating systems

Linux

### What versions of the operating systems?

Debian/bookworm

### Architectures

x86_64

### Stacktrace

_No response_

### Plug-in formats (if applicable)

_No response_

### Plug-in host applications (DAWs) (if applicable)

_No response_

### Testing on the `develop` branch

I have not tested against the `develop` branch

### Code of Conduct

- [X] I agree to follow the Code of Conduct