Remove unnecessary checkout, extract values to secrets

[gregkos]

Checking out the code is not necessary to trigger the deployment via URL, as the code is not used or transferred.

Not sure if the same applies to the API trigger, so I did not remove that, but I would assume it does.

Also extracted some values to GitHub secrets, to encourage best practices.

[jbrooksuk]

Thanks! You can remove the check out from both sections :)

[gregkos]

Thanks! You can remove the check out from both sections :)

Done!

[jbrooksuk]

Thanks!