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