Reconfigured mason for nixos

Mason was failing previously on nixos because it couldn't install the
LSPs it wanted due to how NixOS works. As a workaround, i have tweaked
it (based on williamboman/mason.nvim#428) so
that it attempts to look for LSPs installed on the system before
installing its own version. This prevents the issue without requiring me
to remove mason completely (which i want for non-NixOS systems)

home-manager/dotfiles/.config/nvim/lua/plugins/init.lua

@@ -276,6 +276,11 @@ return {
             --    That is to say, every time a new file is opened that is associated with
             --    an lsp (for example, opening `main.rs` is associated with `rust_analyzer`) this
             --    function will be executed to configure the current buffer
+
+            require("mason").setup {
+                PATH = "append",
+            }
+
             vim.api.nvim_create_autocmd("LspAttach", {
                 group = vim.api.nvim_create_augroup("kickstart-lsp-attach", { clear = true }),
                 callback = function(event)