Preserve SYSTEMTEST_NO_CLEAN when run.sh calls make.
This restores legacy behavior in bin/tests/system where running: SYSTEMTEST_NO_CLEAN=1 ./run.sh would run the test and preserve the output files.
This has been broken since the change that has run.sh invoke "make", due to SYSTEMTEST_NO_CLEAN not being preserved in the environment that's set up for "make".
Another option would be to completely remove SYSTEMTEST_NO_CLEAN.
This seems to be the only behavior-changing environment variable not accounted for in the call to "make".
I don't think this needs a CHANGES entry.
Closes #2662 (closed)