[POC] StageX release builds

[tobtoht]

This is just an experiment for now. I'm still evaluating whether StageX can (eventually) replace Guix for release builds. I will expand on the pros and cons as this PR reaches maturity.

This currently uses scaffolding from Guix builds, which can be cleaned up.

GCC: 13.1.0
Clang: 20.1.8

Target	Builds
x86_64-linux-musl	✓
aarch64-linux-musl	✖
riscv64-linux-musl	✖
x86_64-w64-mingw32	✖
x86_64-unknown-freebsd	✓
x86_64-apple-darwin	✓
arm64-apple-darwin	✓
aarch64-linux-android	✓

What we need:

• mingw-w64 toolchain to cross-compile for Windows
• sysroots or toolchain to cross-compile for aarch64 & riscv64 linux

Todo (misc):

• invalidate depends cache on image change

Comparison (incomplete):

	Guix	StageX
Container Filesystem	1599 MB	349 MB
CI Runtime	1h 26m	17m
Packages (excluding bootstrap)	147	44
Packages (environment)	67	21