ci: add aarch64-unknown-linux-musl release artifact

[malt3]

Fixes #5690.

I tested the workflow on my fork:

• CI run
• test release

The resulting artifact works as expected:

$ file coreutils
coreutils: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, with debug_info, not stripped

$ ./coreutils
coreutils 0.0.23 (multi-call binary)

Usage: coreutils [function [arguments...]]

Currently defined functions:

    [, arch, b2sum, b3sum, base32, base64, basename, basenc, cat, chgrp, chmod, chown, chroot,
    cksum, comm, cp, csplit, cut, date, dd, df, dir, dircolors, dirname, du, echo, env, expand,
    expr, factor, false, fmt, fold, groups, hashsum, head, hostid, hostname, id, install,
    join, kill, link, ln, logname, ls, md5sum, mkdir, mkfifo, mknod, mktemp, more, mv, nice,
    nl, nohup, nproc, numfmt, od, paste, pathchk, pr, printenv, printf, ptx, pwd, readlink,
    realpath, rm, rmdir, seq, sha1sum, sha224sum, sha256sum, sha3-224sum, sha3-256sum, sha3-
    384sum, sha3-512sum, sha384sum, sha3sum, sha512sum, shake128sum, shake256sum, shred, shuf,
    sleep, sort, split, stat, stty, sum, sync, tac, tail, tee, test, timeout, touch, tr, true,
    truncate, tsort, tty, uname, unexpand, uniq, unlink, vdir, wc, whoami, yes

[sylvestre]

Thanks