fix: add libxt6

[dpprdan]

This adds libxt6 and fixes #731 (and rocker-org/devcontainer-templates#28).
According to my tests, it adds roughly 0.67 MB to the r-ver image.

[cboettig]

yeah, I agree the warning messages are annoying here. Though I think ideally upstream packages could address this by handle a soft dependency better, I'm 👍 on merging this.

[eddelbuettel]

But @cboettig consider that this is 100% not what R does. It is either built with x11, or without. Building with, and then withdrawing run-time dependencies, was always a little wild in my eyes. It's all complicated because these have no x11 display but it is also known that certain font calculations etc require x11 mechanics. (I forget what percentage of packages need that but it is also why the xvfb-run wrapper trick for 'virtual' x11 in a virtual framebuffer when needed.)

So yes, not my stack, but I also think that should be put back. It is a really ugly UI wart otherwise.

[cboettig]

@eddelbuettel thanks, excellent point, sounds like you're in favor too so I'll merge.