ci: Test ninja dist in meson jobs

While this isn't critical yet, it might become so at some point, so
let's make sure it keeps working.

Don't set the PIGLIT/XTEST_DIR variables for ninja dist, testing XTS
once against each DDX is enough (as part of ninja test).

Requires git in the docker image.

v2:
* Drop /usr/local/bin/xkbcomp symlink, no longer needed with xkbcomp
  1.4.1 (Peter Hutterer)
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