FIX: remove double quotes in github actions

[12rambau]

The latest prerelease run failed because of an error in the yaml file: https://github.com/pydata/pydata-sphinx-theme/actions/runs/3458681079

searching on the web, I found out that even though double quotes are legit in a yaml file, they are not currently supported by github actions

• Conditional on a job fails if double quotes are used actions/runner#866
• https://docs.github.com/en/actions/learn-github-actions/expressions#:~:text=you%20must%20use%20single%20quotes.%20escape%20literal%20single-quotes%20with%20a%20single%20quote.

specifically:

You must use single quotes. Escape literal single quotes with a single quote.

myString: ${{ 'Mona the Octocat' }}

In order to avoid confusion for the next contributors I decided to remove all the unnecessary double quotes.