Fix venv_adjust.sh sed command for BSD sed (macOS) compatibility

Problem

The evergreen/functions/venv_adjust.sh script fails on macOS with Python 3.13 during venv adjustment:

sed: -e: No such file or directory

Root Cause

BSD sed (macOS) requires an explicit backup extension argument after sed -i, even if empty. The script uses sed -i -e with multiple -e flags, causing BSD sed to misinterpret the first -e as a backup extension. This worked before because the old code used only one -e flag. Python 3.13's cygpath format in activate scripts requires multiple patterns, exposing the bug.