PYTHONSTARTUP is not injected when terminal env experiment is off

[anthonykim1]

When terminal env experiment is off , PYTHONSTARTUP seems to be not injected, which is a bug.
Shell integration for python should work depending on the setting regardless of any other setting or experiment other than upstream terminal shell integration setting.

Update: It was getting injected, but was just getting cleared as part of check:

• We clear env var collection when user isn't in env var experiment to get rid of stale env var.

[anthonykim1]

Verification steps MacOS and Non-WSL linux ONLY:

1. Opt out of terminal env experiment with Python extension via "python.experiments.optOutFrom": [ "pythonTerminalEnvVarActivation" ],
2. Enable shell integration via: terminal.integrated.shellIntegration.enabled
3. Enable Python shell integration via python.terminal.shellIntegration.enabled
4. Type env in your shell (such as zsh), make sure sure there is an environment entry of PYTHONSTARTUP as one of the variables.
5. Launch Python REPL via typing python or python3
6. Make sure shell integration works on REPL. (You see blue, red dots depending on the commands you type in the REPL)