job script: export CYLC_TASK_SUBMIT_NUMBER

New environment variable for the submit number.

(The new test attempts to prove that the submit number is not the same as the try number.)
Can you do an explicit test here?  Believe it or not, under bash-4.2.0 ```(( 1 > 1 )))``` does not cause an abort with ```set -e```.  It does under bash-4.1.2, the other version I have access to at the moment.