gitlab CI: Build against the X server from git

This sits on top of !5 (merged) and is prep work to pass a pipeline for !1 (merged)

We "steal" the container image from the xserver build itself, build + install the X server from git and then build against that installed image. Note that that image is private, so changing the server's CI may break ours.

A better approach would be to install the xserver as part of the server's CI pipeline and then notify XIT to re-run itself against the server. This is what gstreamer does for plugins (and was the motivation for the FDO_BASE_IMAGE variable in ci-templates). But until we get to that point and we have XIT actually run tests as part of the pipeline, let's just do the simplest approach.

cc @p12tic

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