export PYTHONDONTWRITEBYTECODE instead of just defining constant

BUG=chromium:1040314
TEST=shellcheck
Change-Id: I237df73daad84aeebfc934c9ddcc3eb632598827
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1995993Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Gregory Nisbet <gregorynisbet@google.com>

python_runner.sh

@@ -12,7 +12,7 @@
 ## ./git-foo-command  #=> runs `python git_foo_command.py`
 
 ## Constants
-PYTHONDONTWRITEBYTECODE=1
+PYTHONDONTWRITEBYTECODE=1; export PYTHONDONTWRITEBYTECODE
 
 ## "Input parameters".
 # If set before the script is sourced, then we'll use the pre-set values.