gitlab-ci: Set arm job CCACHE_DIR properly

gitlab-ci: Set arm job CCACHE_DIR properly

$PWD doesn't work for variables:, it ended up as "/ccache", always
starting with an empty cache.

v2:
* Use relative path and realpath
v3:
* Use $CI_PROJECT_DIR (Eric Anholt)
* Clear ccache stats in before_script if the cache is in $CI_PROJECT_DIR

Fixes: c9df92bf795a "ci: Switch over to an autoscaling GKE cluster for
                     builds."
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>