CI: open the pr after release-plz release finished

[marcoieni]

Similar to release-plz/release-plz#990

Automatically open the pr to update release-plz version in this repo after release-plz release finished.

tag me in the body so I get the notification.

[marcoieni]

This also avoids users running cargo install release-plz on their CI. They won't need to do this because the artifacts will be available.

Example: https://github.com/LNSD/sops-exec-action/blob/21428d013a5c079f5dac1afda8b17bd180d24780/.github/renovate.json5#L30

https://github.com/LNSD/sops-exec-action/blob/21428d013a5c079f5dac1afda8b17bd180d24780/.github/workflows/test.yaml#L13

Another automation (unrelated): https://github.com/LNSD/sops-exec-action/blob/main/.github/workflows/release.yaml