fix: regex for cran link

[akselthomsen]

Fixes the following:

1. The lychee linter correctly ignores the URLs that are dependent on system variables, which means they cannot be checked.
2. The megalinter workflow now dynamically downloads the right configuration files to make it easier to check PRs such as this one:
   1. If running in this repo it will fetch all downloads from the branch causing the workflow to start
   2. For all other repos it uses the main branch

[github-actions]

Code coverage

Name	Coverage (%)
r.workflows	100
R/hello.R	100

[github-actions]

Github pages

Review the pkgdown webpage for the PR here

[rszymanski]

Approved! 🎉