nvim: Only enable the integration in the carbon repo (#7995)

This fixes an error that would happen when opening neovim in a
non-git-controlled directory, where `root` is `nil` and therefore one
cannot `..` it as a string.

utils/nvim/carbon.lua

@@ -19,8 +19,20 @@ function find_git_ancestor(startpath)
 end
 local root = find_git_ancestor(vim.env.PWD);
 
-vim.lsp.config("carbon", {
-  cmd = { root .. "/bazel-bin/toolchain/carbon", "language-server" },
-  filetypes = { "carbon" },
-})
-vim.lsp.enable("carbon")
+toolchain_path = nil
+if root and vim.uv.fs_stat(root .. "/bazel/carbon_rules") then
+  -- when we're in the carbon repo, use the toolchain from there
+  -- (note that this won't update if one `cd`s into the carbon repo)
+  toolchain_path = root .. "/bazel-bin/toolchain/carbon"
+elseif vim.fn.executable("carbon") then
+  -- otherwise, check if it's on the path
+  toolchain_path = "carbon"
+end
+
+if toolchain_path then
+  vim.lsp.config("carbon", {
+    cmd = { toolchain_path, "language-server" },
+    filetypes = { "carbon" },
+  })
+  vim.lsp.enable("carbon")
+end