oe-build-perf-report-email.py: use pwd for getting user name

oe-build-perf-report-email.py: use pwd for getting user name

Use pwd data instead of os.getlogin() to get the username for fallback
email address. os.getlogin() basically returns the name of the user
logged in on the controlling terminal of the process and raises an
exception on some systems if the process does not have a controlling
terminal - when executed from a cron script, for example.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>