Add some way to set the `entryPoint` in the injector to a custom path

[lyssieth]

Describe the feature request

Currently, the entryPoint used for injection is based on the CONFIG_PATH, which in turn comes from XDG_CONFIG_HOME or from building a path based on other factors.

I would like some way (environment variable, flag, etc) to set the path manually, avoiding the config path debacle.

Why should this be added?

I made a nix flake for Replugged (https://github.com/lyssieth/replugged-nix-flake), and I'm currently having to patch this line in injector.mts to go to a separate location, since a nix build can't (to my knowledge) put files into ~/.config and such.
The patch I'm using is here: https://github.com/lyssieth/replugged-nix-flake/blob/main/patches/replugged-injector.patch

Other than making Nix packaging require less work on my side, I can't think of any other reasons.

Additional information

No response

Please confirm the following

• This is a feature request for Replugged itself, not a feature request for a plugin or theme
• I searched the issues to make sure this feature has not been requested before.