crytsetup: allow overriding the token .so library path via an env var

crytsetup: allow overriding the token .so library path via an env var

I tried to get something similar upstream:

https://gitlab.com/cryptsetup/cryptsetup/-/issues/846

But no luck, it was suggested I use ELF interposition instead. Hence,
let's do so (but not via ugly LD_PRELOAD, but simply by overriding the
relevant symbol natively in our own code).

This makes debugging tokens a ton easier.