Remove use of 'patfix' service connection

[idg10]

Due to a bug in Azure DevOps, the service connection we want to use for accessing GitHub doesn't work, and we've had to introduce a temporary endjin-github-patfix one as a workaround. We should revert to endjin-github as soon as possible.

Ideally we shouldn't even refer to the real service connection names at all in the YAML. But annoyingly, it seems we cannot fully parameterize the service connections used in build pipelines. Of the three variable reference types defined at https://docs.microsoft.com/en-us/azure/devops/pipelines/process/variables?view=azure-devops&tabs=yaml%2Cbatch#understand-variable-syntax two of them (macro and runtime expression) are evaluated too late to be used in template entries, and unfortunately, the "template expression" syntax is not supported for use as the endpoint setting of a repository. This means you are forced to put the actual name of the service connections into certain parts of the YAML.