Option to use the proton entrypoint for protontricks-launch

[Skirmisher]

At the moment, running a Windows executable through protontricks-launch does not invoke the proton Python script, instead taking a route through a custom wine wrapper script that sets up some of the environment itself. This means configuration that Proton normally sets up is not done, and the convenient PROTON_* environment variables can't be used to make any changes.

Most notably, I was recently trying to narrow down an issue that cropped up in a modified game (launched through Protontricks) but didn't appear in the original game (launched through Steam). It turned out the modified game was using wined3d instead of DXVK, due to the proton script not being run!

It's of course possible to invoke proton manually, but Protontricks also doesn't set a couple of environment variables it needs, so a fully-functional command line ends up looking like this:

protontricks -c 'STEAM_COMPAT_CLIENT_INSTALL_PATH="~/.local/share/Steam/legacycompat" STEAM_COMPAT_DATA_PATH="$(realpath "$WINEPREFIX/..")" "$PROTON_PATH/proton" waitforexitandrun COMMAND...' APPID

It would be ideal if protontricks-launch supported an argument to do this itself, or at least set the necessary STEAM_COMPAT_* variables to reduce boilerplate in the invocation.