ci: fetch color.h from the form-packages mirror

[jodavies]

Timeouts fetching color.h from nikhef.nl are causing problems every time the formlib cache needs to be updated.

• Fetch form packages from a mirror: https://github.com/form-dev/form-packages
• Add a -v1 tag to the cache key, which can be incremented when new files are added, so we don't need to remove the existing cache through the github UI.

[coveralls]

coverage: 50.605% (+0.002%) from 50.603%
when pulling 26c46cd on jodavies:formlib
into d9a3230 on form-dev:master.