briefcase dev: Don't overwrite existing environment variables

[NMertsch]

briefcase dev: Only set PYTHONUNBUFFERED=1, PYTHONDEVMODE=1, and PYTHONUTF8=1 if these variables are not already set.

#2497 is only about PYTHONDEVMODE, but I think this generic implementation is less surprising to users. I might lack context, though.

(I fixed two very minor typos along the way, but don't think this is worth a towncrier fragment.)

Fixes #2497

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Thanks - this looks great. The only thing it's missing is documentation of the new feature. A new section on the command reference for dev would make sense to me.

[NMertsch]

Thanks - this looks great. The only thing it's missing is documentation of the new feature. A new section on the command reference for dev would make sense to me.

Thank you. I added the section in commit c97b096, deployed by CI here (once flathub decides to work again). Please let me know if you want any changes (or add them yourself if that feels easier).

[freakboy3742]

Thanks for that - this looks great. Thanks for the PR!