Fix: Use NEXUS_TARGET_BUILD in Nexus IQ CLI scanner

Replace Nexus IQ build Target from "${REQUIREMENTS_FILE}"
to "${NEXUS_TARGET_BUILD}".
The scanner is only including the requirements.txt
file in its scan which should not contain other information
than python package requirements.

Instead, use a "${NEXUS_TARGET_BUILD}" parameter which the
user can optionally provide to the scanner to indicate a
file or directory to include in the scan. By default, this
variable is **/*.

For more information on how to use the "Target" parameter:
https://help.sonatype.com/iqserver/integrations/nexus-iq-cli#NexusIQCLI

Issue: RELENG-4530
Signed-off-by: Jessica Wagantall <jwagantall@linuxfoundation.org>
Change-Id: Ifb6947de3d0ff282d461b12332e6d4c2f4ac0198