test: use T.Setenv to set env vars in tests

[Juneezee]

- What I did
A testing cleanup.

This PR replaces os.Setenv with t.Setenv. Starting from Go 1.17, we can use t.Setenv to set environment variable in test. The environment variable is automatically restored to its original value when the test and all its subtests complete.

Reference: https://pkg.go.dev/testing#T.Setenv

- How I did it

Search for usages of os.Setenv in *_test.go using GoLand, and replace them with t.Setenv accordingly.

- How to verify it

All tests should continue to pass, except those that are already failing before this PR.

- Description for the changelog

test: use `T.SetEnv` to set env vars in tests

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

LGTM

[cpuguy83]

LGTM

[tianon]

This seems fine to me too -- it will potentially cause issues with some packagers who will have to backport Go 1.17 if they want to backport newer Docker releases, but I don't think there's much more we can do (reasonably) to help there.

[thaJeztah]

Thanks @Juneezee !

As Tianon mentioned in the previous comment; we discussed this change in our maintainers call, and agreed that it's ok to make these changes (and it's nice to use more of Go's built-in functionality).

As a follow-up we should have a look at existing uses of gotest.tools env.Patch(), which can now be replaced by t.SetEnv() (and has been deprecated recently in favour of that); https://github.com/gotestyourself/gotest.tools/blob/9c6ef764e11bed6bc0d9455a775d8cac665e1d1d/env/env.go#L18-L26

(We'd still need to keep / want gotest.tools env.PatchAll())

Let me know if you're interested to work on that!

[Juneezee]

Thanks @Juneezee !

As Tianon mentioned in the previous comment; we discussed this change in our maintainers call, and agreed that it's ok to make these changes (and it's nice to use more of Go's built-in functionality).

As a follow-up we should have a look at existing uses of gotest.tools env.Patch(), which can now be replaced by t.SetEnv() (and has been deprecated recently in favour of that); https://github.com/gotestyourself/gotest.tools/blob/9c6ef764e11bed6bc0d9455a775d8cac665e1d1d/env/env.go#L18-L26

(We'd still need to keep / want gotest.tools env.PatchAll())

Let me know if you're interested to work on that!

@thaJeztah Thanks for accepting the PR!

Sure, I'd love to work on replacing env.Patch() as well

[thaJeztah]

oh! I just realised I was touching much of those in #43340, so perhaps I should just include that change in that one 😅