ci: Fix test-docs-ci-conda job to account for external changes

Back in December, the file environment.yml was renamed to conda.yml in
our docs.nextstrain.org repo.¹

¹ <nextstrain/docs.nextstrain.org#175>
  <nextstrain/docs.nextstrain.org@19433c5>

.github/workflows/ci.yaml

@@ -38,7 +38,7 @@ jobs:
     with:
       repo: nextstrain/docs.nextstrain.org
       docs-directory: .
-      environment-file: environment.yml
+      environment-file: conda.yml
 
   test-docs-ci-pip:
     uses: ./.github/workflows/docs-ci.yaml