powershell: fix prepend for new variables and fix unsetenv for non-existing variables

[maxnbk]

Closes #1476
Supercedes PR by @SitiSchu on #1434 (but includes his commit cherrypicked to maintain history/credit)

The test, and commits, also address the same issue but in the "unset" logic, because that too is a victim of this same issue, which is validated by the test, and was in fact the reason I came complaining to @instinct-vfx about not understanding something I was seeing, I just wasn't bright enough to understand it right away.

[JeanChristopheMorinPerso]

@maxnbk FYI, since you are admin on the repo, you could have pushed your commit into @SitiSchu 's PR.

[maxnbk]

@maxnbk FYI, since you are admin on the repo, you could have pushed your commit into @SitiSchu 's PR.

How? The branch exists on their fork.

[JeanChristopheMorinPerso]

If the contributor checks the "Allow maintainers to edit PR", the contributor grants the admins/maintainers access to push to their branch in their fork. See https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork#enabling-repository-maintainer-permissions-on-existing-pull-requests.

On user-owned forks, if you want to allow anyone with push access to the upstream repository to make changes to your pull request, select Allow edits from maintainers.

[maxnbk]

If the contributor checks the "Allow maintainers to edit PR", the contributor grants the admins/maintainers access to push to their branch in their fork. See https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork#enabling-repository-maintainer-permissions-on-existing-pull-requests.

On user-owned forks, if you want to allow anyone with push access to the upstream repository to make changes to your pull request, select Allow edits from maintainers.

Gotcha, good to know.

[JeanChristopheMorinPerso]

LGTM! Thanks @maxnbk and @SitiSchu !

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[JeanChristopheMorinPerso]

Thanks @maxnbk and @SitiSchu !

[instinct-vfx]

LGTM looks good. Thanks @SitiSchu and @maxnbk