fix(ci): swap branches to generate errors references in Nightly GitHub action (#13209)

Co-authored-by: sarah11918 <[email protected]>
Co-authored-by: delucis <[email protected]>

Author: 3 people

.github/workflows/nightly.yml

@@ -62,13 +62,14 @@ jobs:
       matrix:
         include:
           # Default options for running with latest Astro code
-          - SOURCE_BRANCH: main
+          # TODO: switch this back to main source branch for normal operations after v6 merge!
+          - SOURCE_BRANCH: 5-legacy
             TARGET_BRANCH: main
             LABEL: ci
             PR_BRANCH: ci/docgen-errors
             PR_TITLE: 'ci: update error reference docs'
             # Custom options for running with Astro v6 beta code
-          - SOURCE_BRANCH: next
+          - SOURCE_BRANCH: main
             TARGET_BRANCH: v6
             LABEL: 6.0,ci
             PR_BRANCH: ci/docgen-errors-beta