Adding some note about local vs. global nix.conf (input-output-hk#1924)

I faced some puzzling behaviour where artifacts expected to be cached were not found and nix built them. It turned out the local configuration was wrong. It's not clear how this configuration was written in the first place as the most likely candidate I checked should have had different caches.

Author: Arnaud Bailly

docs/troubleshooting.md

@@ -15,7 +15,14 @@ If you think you are doing all of these and you still find you're building GHC,
 ### Why am I building lots of Haskell packages?
 
 We don't generally cache much of Hackage (there's a lot of it!), except for the parts which are used by our tests.
-So this is expected, unfortunately.
+So this is expected, unfortunately. However, if you are building from a recent branch without significant dependencies 
+changes, most of those dependencies should be cached and you should not be building core things like `Cabal` or `ghc-lib`.
+
+If this happens, chances are nix cache are either:
+* Incorrectly configured: Check the aforementioned links,
+* Overriding global configuration in `/etc/nix/nix.conf` with local `~/.config/nix/nix.conf`. This can happen if for example
+  one project is using `nix flake` with some locally configured repositories, while another project is not. 
+  Use `nix show-config` to check which caches are defined, then update or remove `~/.config/nix/nix.conf`.
 
 ### How do I prevent the evaluation-time dependencies of my project from being garbage-collected?