script/build-with-container: optionally source WITH_CRIMSON from env file

script/build-with-container: optionally source WITH_CRIMSON from env file

Add support for optionally sourcing WITH_CRIMSON from the env file that
can be passed to BWC on the command line. When auto-detecting the
crimson variant we previously only looked at the BWC processes
environment. After speaking with Zack we determined that the Jenkinsfile
only writes the WITH_CRIMSON param into the env file, so we add support
to "peek" in the env file for the WITH_CRIMSON variable.

Signed-off-by: John Mulligan <jmulligan@redhat.com>