fix(dependencies): change payjoin version specifier `1.0.0-rc.1` to `=1.0.0-rc.1`

fix(dependencies): change payjoin version specifier `1.0.0-rc.1` to `=1.0.0-rc.1`

Cargo's version resolution always updates to the latest patch version.
If that patch introduces breaking changes, it breaks your application.

This happened after Payjoin released version 1.0.0-rc.2. Because the
version specified for bdk-cli was 1.0.0-rc.1, a fresh install
automatically updated to the newer patch.

To resolve this without migrating to the latest Payjoin API, and to
prevent future breakages from subsequent 1.0.0-rc.x releases, this
commit pins the Payjoin version strictly to =1.0.0-rc.1.