flutter/update: add .in extensions to Nix expression template files

Use autotools inspired extensions (as syntax sort of matches) to make
clear that these are not Nix expressions yet and don't necessarily parse
or evaluate.

pkgs/development/compilers/flutter/update/update.py

···
 
 
 def load_code(name, **kwargs):
-    with open(f"{NIXPKGS_ROOT}/pkgs/development/compilers/flutter/update/{name}", 'r') as f:
+    with open(f"{NIXPKGS_ROOT}/pkgs/development/compilers/flutter/update/{name}.in", 'r') as f:
         code = f.read()
 
     for (key, value) in kwargs.items():