configuration.nix gives many warnings of the form:
substituteStream(): WARNING: pattern <pat> doesn't match anything in file <pat>
<pat> is one of
/usr/bin/readlink and all of the
<files> are from
udev-rules. I believe it started happening after upgrading to 21.05 (or maybe 20.09). The warnings appear to be innocuous. Why does the warning exist and shouldn’t it removed?