Use DRONE_SOURCE_BRANCH instead of DRONE_COMMIT_REFSPEC

Looks like DRONE_COMMIT_REFSPEC is not available anymore on Drone 1.x

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>

build/ca-bundle-checker.sh

 #!/usr/bin/env bash
 
-printenv
-
-if [[ -n ${DRONE_COMMIT_REFSPEC} &&  ! ${DRONE_COMMIT_REFSPEC} =~ version(\/noid)?\/([0-9.]+) ]]; then
+if [[ -n ${DRONE_SOURCE_BRANCH} &&  ! ${DRONE_SOURCE_BRANCH} =~ version(\/noid)?\/([0-9.]+) ]]; then
     echo "Skip CA bundle check"
     exit 0
 fi