generate_report.sh: Use Jenkins ennvar for default builder job name

This makes it compatible with personal staging job names (e.g.
pfalcon-tf-a-builder).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I6b2e5b0c661bd6c8a09b624bb090b2d5e0bb33cb
1 file changed