buildenv: Limit exclusion of info/dir

While trying to exclude a generated `/share/info/dir` in a package
from symlinking into a profile, we also ended up excluding the Emacs
info manual for `dired-x`. This change excludes only the dir file.

pkgs/build-support/buildenv/builder.pl

···
     return if
         $relName eq "/propagated-build-inputs" ||
         $relName eq "/nix-support" ||
-        $relName =~ /info\/dir/ ||
+        $relName =~ /info\/dir$/ ||
         ( $relName =~ /^\/share\/mime\// && !( $relName =~ /^\/share\/mime\/packages/ ) ) ||
         $baseName eq "perllocal.pod" ||
         $baseName eq "log" ||