post_process: clearly delineate the beginning os script output.

This is a companion change to
https://review.spdk.io/gerrit/c/build_pool/jenkins_build_pool/+/1810


and the two should be merged together to make sure the entire output
of the post process script is included in one file when output from
the CI.

Signed-off-by: Seth Howell <seth.howell@intel.com>
Change-Id: Ia6506208881f5d745ff63a356d3965426449ecc7
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1811


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>