chore(nix): move rustSimple from users.Profpatsch.writers

I think it’s solid enough to use in a wider context.

Change-Id: If53e8bbb6b90fa88d73fb42730db470e822ea182
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3055
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: lukegb <lukegb@tvl.fyi>

users/Profpatsch/netstring/tests/default.nix

@@ -2,15 +2,10 @@
 
 let
 
-  inherit (depot.users.Profpatsch.netstring)
-    python-netstring
-    rust-netstring
-    ;
-
   python-netstring-test = depot.users.Profpatsch.writers.python3 {
     name = "python-netstring-test";
     libraries = p: [
-      python-netstring
+      depot.users.Profpatsch.netstring.python-netstring
     ];
   } ''
     import netstring
@@ -38,10 +33,10 @@ let
     )
   '';
 
-  rust-netstring-test = depot.users.Profpatsch.writers.rustSimple {
+  rust-netstring-test = depot.nix.writers.rustSimple {
     name = "rust-netstring-test";
     dependencies = [
-      rust-netstring
+      depot.users.Profpatsch.netstring.rust-netstring
     ];
   } ''
     extern crate netstring;