environment-variables: fix APPVEYOR_REPO_PROVIDER value for GitLab and GitHub
environment-variables: fix APPVEYOR_REPO_PROVIDER value for GitLab and GitHub

They were listed as `gitlab`/`github`, but in reality they are `gitLab`/`gitHub` (with an uppercase `L`/`H`) respectively.

Possibly `bitbucket` is also affected, but I haven't tested it.
appveyor/website,appveyor/website,appveyor/website