Allow customizing output directory for generated assets

[tranzystorekk]

Makes packaging easier, e.g. by doing export BAT_ASSETS_GEN_DIR=${PWD}/gen instead of looking in the target dir.

[sharkdp]

But... you can already do that using OUT_DIR, no?

[tranzystorekk]

Not really, that's an env var that cargo sets for the build script, overwriting anything from the current shell

[sharkdp]

Ah, right. Thank you for the clarification. In that case, this looks good.