ci: fix building the search index

[radoering]

Building the search index fails since the hugo update in #159:

• last successful run: https://github.com/python-poetry/website/actions/runs/12076967191
• first failure: https://github.com/python-poetry/website/actions/runs/12082967521

The failure is caused by hugo-bin (since v0.102.0) removing the vendor directory, which is created by bundle install while setting up ruby, during installation: fenneclab/hugo-bin@v0.101.5...v0.102.0

The following runs of a temporarily changed Deploy Preview workflow show that this PR fixes the issue (the index is normally only built in the Deploy Production workflow):

• without fix: https://github.com/python-poetry/website/actions/runs/12569633498
• with fix: https://github.com/python-poetry/website/actions/runs/12571042788