Fix URL of the 'View App' button in MRs

Commit edeb00a5 did not fix this, instead it made the button not work at all.

Previous, non working URL:

• https://kicad.gitlab.io/libraries/kicad-footprints/-/kicad-footprints/-/jobs/6253161458/artifacts/diffs/index.html.diff/index.html

URL where the diff actually exists

• https://kicad.gitlab.io/-/libraries/kicad-footprints/-/jobs/6253161458/artifacts/diffs/index.html.diff/index.html