ci: update default flux-security version to v0.8.0

Problem: The default FLUX_SECURITY_VERSION used in docker-run-checks.sh
is v0.7.0, but there is v0.8.0 version of flux-security that should
be used in testing.

Update the default FLUX_SECURITY_VERSION to 0.8.0.

Author: grondo

src/test/docker/docker-run-checks.sh

@@ -16,7 +16,7 @@ JOBS=2
 MOUNT_HOME_ARGS="--volume=$HOME:/home/$USER -e HOME"
 
 if test "$PROJECT" = "flux-core"; then
-  FLUX_SECURITY_VERSION=0.7.0
+  FLUX_SECURITY_VERSION=0.8.0
   POISON=t
 fi