feat: adding pkgs support for cross-compilation

[SilentVoid13]

closes #160.

This should not introduce any breaking changes, we can now pass a buildInputs and nativeBuildInputs lambda to craneMultiBuild that we get resolved in mkStdToolchains and ultimately passed to craneLib in mkFenixToolchain.

EDIT: I just realized my new commits were also forwarded to this PR, they basically add new targets (linux musl variants + windows64 with mingw).

[dpc]

Is this really needed? I thought we'd just modify mergeArgs to scan through buildInputs and buildNativeInputs and apply pkgs over functions there.

[SilentVoid13]

I did try the mergeArgs route but it was a pain passing the buildInputs and nativeBuildInputs across different targets.
Resolving the packages as early as possible to pass it directly to mkFenixToolchain ended up working smoothly

[dpc]

We'll need tests in ./checks that make sure it works. But right now that craneArgs looks off to me.

[SilentVoid13]

Got it, thanks for the review. I did not manage to get mergeArgs to work but I'm fairly new to writing nix

[SilentVoid13]

Hey @dpc, coming back to this PR

• just pushed some tests for cross-compilation on new targets
• suppressed cross pkgs for android targets since most packages look broken (e.g. openssl)
• updated clang to version 17 since it looks like this is the version that is picked up now with clang-unwrapped

Tell me if there are still blockers to merge this

[dpc]

I never really had stamina to fully figure it out while it was (AFAICT) slightly off, and personally I did not care about it, so I never did it.

I decided to have a clanker help me with it now: #239