[che-theia] build of docker images should not make GITHUB_TOKEN mandatory

[benoitf]

Is your enhancement related to a problem? Please describe.

When building docker images of Theia, GITHUB_TOKEN is a required parameter of the build script while it shouldn't
For now this parameter is mandatory. Not providing it through ./build.sh --build-arg GITHUB_TOKEN=.... is making the build always fail. It should be an optional parameter that should be mandatory only if we're forced to provide it because for example we share a network with other ppl making a lot of github requests.

Describe the solution you'd like

Be able to build the images without this token.

Also for now the check on a github rate limit is done inside the dockerfile. We should probably move out this code to the build script.

Describe alternatives you've considered

Additional context