[ci] Update `cron-update-rust.yml` workflow

[ValuedMammal]

The Update rust version workflow was failing due to missing app id and related data.

https://github.com/bitcoindevkit/bdk_wallet/actions/runs/15034226862/job/42861284677

[notmandatory]

Fixed, see #242

Steps to fix:

1. Verify "Create PR Actions" app added to https://github.com/bitcoindevkit/bdk_wallet/settings/installations
2. Add the following "Repository Secrets" to https://github.com/bitcoindevkit/bdk_wallet/settings/secrets/actions

• APP_ID: this ID can be found in the "Create PR Actions" app settings
• APP_PRIVATE_KEY: this KEY can be generated from the "Create PR Actions" app
• GPG_PRIVATE_KEY: @ValuedMammal I will give you a copy of this private key

@ValuedMammal please verify you have access to the above GitHub actions app and settings links, if not we should do a screen share so I can get you the right permissions.

[ValuedMammal]

Seems to be fixed now, thanks @notmandatory. No, I wasn't able to access or configure the installed github apps.

[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.