Update "Update rustc" job

[notmandatory]

Looks like the tibdex/github-app-token@v2 action we're using now needs to be replaced with Githubs new offical action that does the same thing: https://github.com/actions/create-github-app-token.

Originally posted by @notmandatory in #228

Relevant to bdk_chain ~~and bdk_wallet~~ repos.