ci: Rework path handling

This patch cleans up the CI path handling, and
allows for external path variables to be used.

Environment variables are now respected for MESA_PATH,
VIRGL_PATH and CTS_PATH.

Additionally these variables can be picked up from a
config file `~/.virgl-ci.conf`

$ cat ~/.virgl-ci.conf
MESA_PATH=/opt/virgl/mesa
VIRGL_PATH=/opt/virgl/virglrenderer

Or given as a local environment variable like below.

CTS_PATH=/opt/virgl/ ci/run_test_suite.sh --piglit --gles3 --backend softpipe

Signed-off-by: Robert Foss <robert.foss@collabora.com>
7 jobs for ci_paths in 53 minutes and 13 seconds (queued for 2 seconds)