chore: Only release-plz after release PRs (#1391)

Stops releases from being created by new packages pushed to `main`.

https://release-plz.ieni.dev/docs/config#the-release_always-field

This saves us from having to temporarily configure `git_tag_enable` and
`git_release_enable` for each crate.
CQCL/tket2@73d6e65


Copied from CQCL/tket2#524

release-plz.toml

@@ -15,6 +15,13 @@ changelog_config = "cliff.toml"
 git_tag_name = "{{ package }}-v{{ version }}"
 git_release_name = "{{ package }}: v{{ version }}"
 
+# Only create releases / push to crates.io after merging a release-please PR.
+# This lets merge new crates to `main` without worrying about accidentally creating
+# github releases.
+#
+# To trigger a release manually, merge a PR from a branch starting with `release-plz-`.
+release_always = false
+
 [[package]]
 name = "hugr"
 changelog_include = ["hugr-core", "hugr-passes"]