ci: Disable egl_ext_device_drm tests in piglit.

Emma Anholt requested to merge anholt/mesa:piglit-no-device-base into master

If the runner has a HW device that would be supported, even without /dev/dri forwarded into the container, it will be enumerated and the tests on llvmpipe fail with (for example):

libEGL warning: Not allowed to force software rendering when API explicitly selects a hardware device. libEGL warning: MESA-LOADER: failed to open i965 (search paths /builds/anholt/mesa/install/lib/dri)

Given that we can't necessarily control the DRI devices present on the runners (particularly for developers bringing their own runners to reduce the demands on fd.o's shared resources), just skip these tests in CI.

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