ci: add release-plz config file (#57)

Attempt to address
#55 (comment)

Maybe what happened is:
1. PR merges, `release_always` is by default true so the `release-plz`
CLI runs
1. `release-pr` command (`update` command AND opening of PR): it tries
to update but failed (e.g. no pkg on crates.io and no CHANGELOG file
yet, see
https://github.com/privacy-scaling-explorations/zk-kit.rust/actions/runs/11124754559/job/30910962049#step:4:230).
A PR is opened nonetheless.
  2. `release` command: runs nonetheless, new crate is published
2. we merge the release PR created by the previous release GH workflow
(previous step)
3. In the next release workflow run, the `release-pr` step that includes
the `update` command detects a new commit (the PR merge at step 1) and
bumps the version again, creating a new PR too.

A fix could be to set
[`release_always`](https://release-plz.ieni.dev/docs/config#the-release_always-field)
to `false`.

.release-plz.toml

@@ -0,0 +1,2 @@
+[workspace]
+release_always = false