Don't import impure overlays if meta.nixpkgs is a path

Also update documentations to recommend specifying `overlays = []`
explicitly.

Fixes #39.

manual/src/release-notes.md

@@ -5,6 +5,7 @@
 - `--reboot` is added to trigger a reboot and wait for the node to come back up.
 - The target user is no longer explicitly set when `deployment.targetUser` is null ([#91](https://github.com/zhaofengli/colmena/pull/91)).
 - In `apply-local`, we now only escalate privileges during activation ([#85](https://github.com/zhaofengli/colmena/issues/85)).
+- Impure overlays are no longer imported by default if a path is specified in `meta.nixpkgs` ([#39](https://github.com/zhaofengli/colmena/issues/39))
 
 ## [Release 0.3.0](https://github.com/zhaofengli/colmena/releases/tag/v0.3.0) (2022/04/27)
 

manual/src/tutorial/flakes.md

@@ -53,6 +53,7 @@ Here is a short example:
       meta = {
         nixpkgs = import nixpkgs {
           system = "x86_64-linux";
+          overlays = [];
         };
       };