home(neovim): add gcc to env

A compiler is required for the compilation of treesitter grammars. I
should probably figure out how to just add every grammar to Neovim, but
this works with my existing configuration for now. I'm choosing to keep
my config separate because I want to still be able to use it on systems
without Nix.

home/programs/neovim.nix

@@ -15,6 +15,7 @@ in
         rust-analyzer
         vscode-langservers-extracted
         pyright
+        gcc
       ];
     };
   };