ci: say which busbar branch we mean, instead of '<number>/merge'

On a pull_request event `github.ref_name` is '<number>/merge', not a branch name,
so plugin-ci asked GetBusbar/busbar for a branch called '5/merge' and the sibling
checkout died with 'The process /usr/bin/git failed with exit code 1', naming
neither the ref nor the reason. Every pull request to this repo has been red on
that before running a line of the change it was testing.

`github.base_ref` is the branch the PR targets and is empty on a push, so this
means dev-against-dev and main-against-main as intended on both events.
plugin-ci.yml also now falls back with a loud warning when a ref cannot be
resolved, so this is belt and braces - but a caller should say what it means
rather than lean on a fallback.

Author: Matthew

.github/workflows/ci.yml

@@ -18,7 +18,10 @@ jobs:
       plugin_crate: busbar-auth-ldap-plugin
       plugin_kind: auth
       plugin_alias: ldap
-      busbar_ref: ${{ github.ref_name }} # same-branch: qa builds core qa, dev builds core dev (no stale main/tag)
+      busbar_ref: ${{ github.base_ref || github.ref_name }}
+      # base_ref FIRST: on a pull_request `github.ref_name` is '<number>/merge', not a branch
+      # name, so this asked busbar for a branch called '5/merge' and the sibling checkout died
+      # with an unreadable git error on EVERY pull request to this repo. # same-branch: qa builds core qa, dev builds core dev (no stale main/tag)
       # OpenLDAP service container for the live credential-form e2e (tests/e2e.rs): the plugin binds
       # alice against it, reads her group DN, mints a key; a wrong password is asserted 401.
       service: openldap