Ensure stat comments only run on PRs

[domoscargin]

Stat comments are part of the tests workflow, which runs on pulls and pushes, but should only run on PRs.

This should hopefully make sure that happens.

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-4.6.0.min.css	118.07 KiB
dist/govuk-frontend-4.6.0.min.js	47.1 KiB
dist/govuk-frontend-ie8-4.6.0.min.css	79.27 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	72.35 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	67.96 KiB
packages/govuk-frontend/dist/govuk/all.mjs	3.91 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	34.31 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.83 KiB

Modules

File	Size
all.mjs	64.5 KiB
components/accordion/accordion.mjs	20.86 KiB
components/button/button.mjs	3.67 KiB
components/character-count/character-count.mjs	20.46 KiB
components/checkboxes/checkboxes.mjs	4.37 KiB
components/error-summary/error-summary.mjs	4.96 KiB
components/exit-this-page/exit-this-page.mjs	15.42 KiB
components/header/header.mjs	2.3 KiB
components/notification-banner/notification-banner.mjs	3.5 KiB
components/radios/radios.mjs	3.37 KiB
components/skip-link/skip-link.mjs	2.12 KiB
components/tabs/tabs.mjs	8.04 KiB

View stats and visualisations on the review app

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Action run for ab2032d

[colinrotherham]

Looks good. This manual run was correctly skipped:
https://github.com/alphagov/govuk-frontend/actions/runs/5962662757

Would be nice for non-PRs runs to write to GitHub workflow status (not PR comment) but one for another day!