Run Sass workflows against built sass files rather than sources.

[romaricpascal]

What

Update our Sass workflows so they run against the build Sass files (in packages/govuk-frontend/dist) rather than the sources as they do now.

This would likely require to update when the workflow runs so it executes after the build has been done and cached (similarly to running Percy), to avoid building the project multiple times.

Why

This will ensure that they run against the code our users will be importing, which is not quite the same as the sources as PostCSS and Autoprefixer will have run, updating the code and adding sourcemaps.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• Sass workflows run against the built package