Add support to build with musl

[fzakaria]

Add support to the flake.nix to build with musl

nix build .#patchelf-musl

ld-musl-x86_64.so.1 --list ./result/bin/patchelf
	/nix/store/2lp9hm9idb2y8zxfqi4nnvxsrk4vym6d-musl-1.2.2/lib/ld-musl-x86_64.so.1 (0x7fc3d791d000)
	libstdc++.so.6 => /nix/store/b29ihq5w7rh1qga809bxsdrh2mrrvd7m-gcc-10.3.0-lib/lib/libstdc++.so.6 (0x7fc3d76f7000)
	libgcc_s.so.1 => /nix/store/b29ihq5w7rh1qga809bxsdrh2mrrvd7m-gcc-10.3.0-lib/lib/libgcc_s.so.1 (0x7fc3d76dd000)
	libc.so => /nix/store/2lp9hm9idb2y8zxfqi4nnvxsrk4vym6d-musl-1.2.2/lib/ld-musl-x86_64.so.1 (0x7fc3d791d000)

./result/bin/patchelf --help
syntax: ./result/bin/patchelf
  [--set-interpreter FILENAME]
...

You can also launch a develop shell for glib and musl.

CC @Mic92

[fzakaria]

I think there is an opportunity here to converge the hydraJobs here too and provide the full matrix of:

LIBC (MUSL vs. GLIBC) x DYNAMIC (static vs dynamic) x ASAN (on/off)

Might be a useful PR after this.