Conditionally use Xvfb

[maleadt]

Start from the list in https://github.com/JuliaCI/PackageEvaluator.jl/blob/d1cc2a42fa3640086b73c274903fec46ab4d1861/src/constants.jl

[vtjnash]

Similarly with Qt plugin "xcb", which appears to be the cause of a dozen or so issues such as
https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_date/2021-02/21/ArviZ.1.7.0-DEV-d524f21917.log

[maleadt]

Looks like libxcb gets installed there using conda, so it should just work? Probably a missing dependency, which we could add to the container used to run these tests (although I've generally not done so now that we're using Artifacts more and more).