Switch build-args to expected string format

I'm not sure why, but build-push-action expects the parameters labeled
as List to be newline separated strings. It does not accept arrays of
strings.

.github/workflows/build-unstable-branch.yml

@@ -28,6 +28,5 @@ jobs:
           tags: 'ghcr.io/mlibrary/dromedary/dromedary-unstable:${{ github.sha }}'
           file: ./Dockerfile
           target: production
-          build-args:
-            - RAILS_RELATIVE_URL_ROOT=/m/middle-english-dictionary
-
+          build-args: |
+            RAILS_RELATIVE_URL_ROOT=/m/middle-english-dictionary