Add PAT for devtools::install_github()

[jeanpaulrsoucy]

Today, the Dockerfile build of ccodwg-update failed due to hitting the GitHub API rate limit during the devtools::install_github step. There are two solutions here:

• Pass a GitHub PAT to the request in the Dockerfile (see --build-arg)
• Move the install_github step to the actual update script and pass the GitHub PAT here instead