setup_venv.sh fails on macos sed

[Elijah Zupancic]

Describe the bug
When running setup_venv.sh on MacOS, the script fails:

Downloading Pulumi CLI into virtual environment
sed: illegal option -- r
usage: sed script [-Ealn] [-i extension] [file ...]
       sed [-Ealn] [-i extension] [-e script] ... [-f script_file] ... [file ...]

Expected behavior
Sed should be invoked with the posix compatible flag -E instead of -r which is specific to GNU sed.

[Jason Schmidt]

Will close with #167