add instructions for generating flake.lock to envrc-flake

Previous setup instructions did not work without. (i.e. the envrc would
not do anything, `nix flake show..` would provide unhelpful error)

src/tools/nix-dev-shell/envrc-flake

@@ -1,7 +1,7 @@
 # If you want to use this as an .envrc file to create a shell with necessery components 
 # to develop rustc, use the following command in the root of the rusr checkout:
 #	
-# ln -s ./src/tools/nix-dev-shell/envrc-flake ./.envrc && echo .envrc >> .git/info/exclude
+# ln -s ./src/tools/nix-dev-shell/envrc-flake ./.envrc && nix flake update --flake ./src/tools/nix-dev-shell && echo .envrc >> .git/info/exclude
 
 if nix flake show path:./src/tools/nix-dev-shell &> /dev/null; then
   use flake path:./src/tools/nix-dev-shell