Include cargo-insta in devShell in nix flake (#3470)

# Description of Changes

For to run `cargo insta test`, `cargo insta review`, &c during local
development.

# API and ABI breaking changes

N/a

# Expected complexity level and risk

1: just changes to the devShell in the flake, not even the actual
package build.

# Testing

- [x] Ran `cargo insta test` locally using the dev shell.

Author: gefjon

flake.nix

@@ -160,7 +160,12 @@
             #                   from src/pac.c:1:
             CFLAGS = "-O";
 
-            packages = [rustStable rustNightly cargoArtifacts];
+            packages = [
+              rustStable
+              rustNightly
+              cargoArtifacts
+              pkgs.cargo-insta
+            ];
           };
         }
     );