node-sass workflow: improve error message when Sass vars are found

[julien-deramond]

Description

This PR suggests improving the error message of the node-sass workflow when we find Sass variables in dist-sass.

In the first commit, I voluntarily reverted some changes in our CSS files in https://github.com/twbs/bootstrap/pull/38448/files to show the new error message.
It can be found here: https://github.com/twbs/bootstrap/actions/runs/4679714004/jobs/8290111973?pr=38448.

Then, the second commit rollbacks the wrong changes so that the workflow is green again.

Extra information: bash {0} is used to avoid "Fail-fast behavior" (See https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#exit-codes-and-error-action-preference)

Motivation & Context

When it's not possible to run npx locally because of security rules in a company or because you don't remember the command line, it can be really handy to have the list of the errors directly in the logs of the failed workflow.

Type of changes

• Enhancement (non-breaking change which adds functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• (N/A) I have updated the documentation accordingly
• (N/A) I have added tests to cover my changes
• All new and existing tests passed