fish: pick up completion files from other packages

Some packages bring their own completions in
/share/fish/vendor_completions.d. Now they are picked up by fish from
every path in NIX_PROFILES.

pkgs/tools/security/pass/default.nix

@@ -44,7 +44,7 @@ stdenv.mkDerivation rec {
   preInstall = ''
     mkdir -p "$out/share/bash-completion/completions"
     mkdir -p "$out/share/zsh/site-functions"
-    mkdir -p "$out/share/fish/completions"
+    mkdir -p "$out/share/fish/vendor_completions.d"
   '';
 
   installFlags = [ "PREFIX=$(out)" ];