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Installed symlinks (specifically ld-musl-x86_64.so.1 have permissions 0700
on MacOS, which means only the owner (typically root) can read them.

Symlink permissions can't be anything but 0777 on Linux, but on Mac they
can be set, and in this case are being inherited from the 077 umask in
install.sh:

```
$ ls -l
/nix/store/fgkznmnz1swzp8ck75fa2zvj62pkjgvq-musl-x86_64-unknown-linux-musl-1.2.3/lib/ld-musl-x86_64.so.1
ls: cannot read symbolic link
'/nix/store/fgkznmnz1swzp8ck75fa2zvj62pkjgvq-musl-x86_64-unknown-linux-musl-1.2.3/lib/ld-musl-x86_64.so.1':
Permission denied
lrwx------ 1 root wheel 7 Jan  1  1970
/nix/store/fgkznmnz1swzp8ck75fa2zvj62pkjgvq-musl-x86_64-unknown-linux-musl-1.2.3/lib/ld-musl-x86_64.so.1
```

My fix (attached) is to use `umask 022`, which was already being used to
make directories. It's not practical to fix this by specifying the intended
permissions for this symlink, as setting link permissions requires the
nonstandard `-h` chmod flag, which presumably fails on other platforms.

First discovered when cross-building on MacOS for linux:
https://github.com/NixOS/nixpkgs/issues/285141

I've tested the fix works in nix. I am fairly confident the same issue
exists outside of Nix given the fix, but I haven't built musl before and
ran into unrelated errors.

Thanks,
 - Tim