rustlings: add gcc to PATH

fixes the following traceback when compiling an exercise:

```bash
$ rustlings run
error: linker `cc` not found
  |
  = note: No such file or directory (os error 2)

error: could not compile `exercises` (bin "variables1") due to 1 previous error
```

pkgs/by-name/ru/rustlings/package.nix

@@ -8,6 +8,7 @@
   cargo,
   rustc,
   clippy,
+  gcc,
   makeWrapper,
 }:
 let
@@ -47,6 +48,7 @@ rustPlatform.buildRustPackage {
         cargo
         rustc
         clippy
+        gcc
       ]
     }
   '';