feat: drop rattler_installs_packages for uv crates

[mattem]

Drops the dependency on rattler_installs_packages in favour of the uv crates.

Note that while this works, the creates of uv do not officially support depending directly on the crates, however there is little overhead here, and is pinned to a fixed SHA.

Closes #307
Closes #344
Closes #345

Due to compilation issues under llvm, this diff also switches to musl for the toolchain binaries. We need to keep llvm for proc macros, unfortunately.

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

Drop dependency on rattler_installs_packages in favour of the uv crates.
Produce statically linked toolchain binaries via musl.

Test plan

• New test cases added