Skip “Percy screenshots” when secrets are unavailable on forks

[colinrotherham]

The Percy CLI requires ${{ secrets.PERCY_TOKEN }} which forks won't necessarily have. We recently made our Percy npm script return an error code when the health check failed in:

• Send Percy screenshots via npm script (not Jest) #3009

This PR adds a condition to our Send screenshots to Percy job which makes it skippable

# Skip when secrets are unavailable on forks
if: ${{ github.repository_owner == 'alphagov' }}

Where permissions allow you'll see we've now added a Dependabot PERCY_TOKEN secret too:
https://github.com/alphagov/govuk-frontend/settings/secrets/dependabot

Reducing Percy runs in future

We may want to consider a "paths" filter in future so this PR also moves the Percy uploads to a separate GitHub Actions .github/workflows/screenshots.yml workflow. More information in:

• Only run visual regression tests if there are code changes that might cause a visual change #2864

[36degrees]

Is there any benefit to having this inside the tests workflow, rather than just having screenshots.yml trigger on the same pull_request and push events?

[colinrotherham]

Yeah, it gives the Install and Build steps chance to get cached

Then when we trigger the Percy workflow it can go super fast (already installed and built)

[36degrees]

Makes sense – but I think this might be worth adding as a comment somewhere as it's not obvious.

[colinrotherham]

Sounds good, comment added

Similar to how we only install js-beautify in the Diff changes to dist workflow it would be nice in future (we're nearly there) to just install Percy with the Review app devDependencies:

npm ci --workspace app