actions/checkout: update input descriptions

In fb32eb0f296812da443357eafaa350031de431d7 we moved the checked-out
worktrees into a `nixpkgs` directory. Update the input descriptions to
reflect this.

Changed files
+2 -2
.github
actions
checkout
+2 -2
.github/actions/checkout/action.yml
···
inputs:
merged-as-untrusted-at:
-
description: "Whether and which SHA to checkout for the merge commit in the ./untrusted folder."
+
description: "Whether and which SHA to checkout for the merge commit in the ./nixpkgs/untrusted folder."
target-as-trusted-at:
-
description: "Whether and which SHA to checkout for the target commit in the ./trusted folder."
+
description: "Whether and which SHA to checkout for the target commit in the ./nixpkgs/trusted folder."
runs:
using: composite