Allow selecting SIM slot using environment variable in gbinder driver

Hi! Several SailfishOS users are adapting and testing lpac for this system and also creating a graphical interface to manage eSIMs using lpac (you can follow this thread on the forum https://forum.sailfishos.org/t/esim-on-xperia-10-iii-and-other-sfos-devices/14962/23?u=juanro49). We have seen that currently the gbinder driver automatically selects the SIM slot in which it detects an eSIM. This is fine, but if you have two eSIMs (integrated eSIM + eSIM with physical SIM card, or two eSIMs with physical SIM cards), it would only detect the one in the first slot, ignoring the one in the second. In order to manage both slots, we have made a modification to the gbinder driver https://github.com/juanro49/lpac/commit/971580ef10a18a66610b37e697fb94f2a446dfba based on what has already been done in https://github.com/estkme-group/lpac/pull/97, so that the slot can be selected using an environment variable (and if it is not specified or -1 is specified, it is done automatically as before). I have created an issue instead of directly a PR because there is a comment in the code that says “we don't want to force the user to choose in this case” and therefore, I suppose it is necessary to discuss whether you want the change (and check that the code style is correct, as I don't usually program in C) before creating the PR.