treewide: use inherit for attribute assignments

This change converts redundant attribute assignments of the form `a =
a;` or `a = someSet.a;` into cleaner `inherit` statements. This reduces
verbosity and follows common Nix style for bringing attributes into
scope.

Statix Codes: W03 (manual_inherit), W04 (manual_inherit_from)

Also include statix and the rule in our configuration.

.github/PULL_REQUEST_TEMPLATE.md

@@ -22,7 +22,7 @@ Also make sure to read the guidelines found at
 - [ ] Change is backwards compatible.
 
 - [ ] Code formatted with `nix fmt` or
-      `nix-shell -p treefmt nixfmt deadnix keep-sorted nixf-diagnose --run treefmt`.
+      `nix-shell -A dev --run treefmt`.
 
 - [ ] Code tested through `nix run .#tests -- test-all` or
       `nix-shell --pure tests -A run.all`.