This PR adjusts the Project: Update Dependency command so that it updates the toolchain after running lake update. This was prompted by the introduction of a post-update hook in leanprover-community/mathlib4#8243 that automatically updates the toolchain.

With the post-update hook and asking to update the toolchain before running lake update, a user decision of "Do not update the toolchain" would be ignored for projects downstream of Mathlib, as the post-update hook will update the toolchain regardless of the user choice. Asking to update the toolchain (if it changed) after lake update ensures users do not see a misleading dialog.

One caveat is that this means that we will now run lake update with the Lean version of the project itself, not the Lean version of its dependency.

Another change induced by switching the order of operations is that we can now read the toolchain from lake-packages instead of fetching it from raw.githubusercontent.com.