nixpkgs/pkgs/build-support/setup-hooks/make-symlinks-relative.sh

postFixupHooks+=(_makeSymlinksRelative)

# For every symlink in $output that refers to another file in $output
# ensure that the symlink is relative. This removes references to the output
# has from the resulting store paths and thus the NAR files.
_makeSymlinksRelative() {
    local symlinkTarget

    if [ -n "${dontRewriteSymlinks-}" ]; then
        return 0
    fi

    while IFS= read -r -d $'\0' f; do
        symlinkTarget=$(readlink "$f")
        if [[ "$symlinkTarget"/ != "$prefix"/* ]]; then
            # skip this symlink as it doesn't point to $prefix
            continue
        fi

        if [ ! -e "$symlinkTarget" ]; then
            echo "the symlink $f is broken, it points to $symlinkTarget (which is missing)"
        fi

        echo "rewriting symlink $f to be relative to $prefix"
        ln -snrf "$symlinkTarget" "$f"

    done < <(find $prefix -type l -print0)
}