pathogen-repo-ci: Support a continue-on-error pass thru

Allows calling workflows to succeed even if the job in this called
workflow fails.  This is a workaround for the calling working being
unable to specify continue-on-error itself.¹

Related-to: <nextstrain/docker-base#148>

¹ <https://docs.github.com/en/actions/using-workflows/reusing-workflows#supported-keywords-for-jobs-that-call-a-reusable-workflow>

.github/workflows/pathogen-repo-ci.yaml

@@ -44,13 +44,21 @@ on:
         default: ""
         required: false
 
+      continue-on-error:
+        description: >-
+          Pass thru for <https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idcontinue-on-error>.
+        type: boolean
+        default: false
+        required: false
+
 permissions:
   contents: read
   packages: read
 
 jobs:
   build:
     runs-on: ubuntu-latest
+    continue-on-error: ${{ inputs.continue-on-error }}
     steps:
       # Log in, if possible, to docker.io (Docker Hub), since authenticated
       # requests get higher rate limits (e.g. for image pulls).  Our org-level