nix: configure sharefs as a mount unit

This should have been configured via fileSystem instead, but that somehow has no effect, it is not even being evaluated, and I am not wasting any more time on nix. This change aims to keep everything working until all nix/systemd tooling is replaced.

Signed-off-by: Ophestra <cat@gensokyo.uk>

options.nix

@@ -41,6 +41,12 @@ in
       };
 
       sharefs = {
+        package = mkOption {
+          type = types.package;
+          default = packages.${pkgs.stdenv.hostPlatform.system}.sharefs;
+          description = "The sharefs package to use.";
+        };
+
         user = mkOption {
           type = types.str;
           default = "sharefs";