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

[Juneezee]

Changes

/kind 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. This ensures that each test does not start with leftover environment variables from previous completed tests.

This saves us at least 2 lines (error check, and unsetting the env var) on every instance.

func TestFoo(t *testing.T) {
	// before
	key := "ENV"
	originalEnv := os.Getenv(key)
	
	if err := os.Setenv(key, "new value"); err != nil {
		t.Fatal(err)
	}
	defer func() {
		if err := os.Setenv(key, originalEnv); err != nil {
			t.Logf("failed to set env %s back to original value: %v", key, err)
		}
	}()
	
	// after
	t.Setenv(key, "new value")
}

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
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  functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

Hi @Juneezee. Thanks for your PR.

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[vdemeester]

/ok-to-test

Thanks @Juneezee

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vdemeester

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[abayer]

/retest

[lbernick]

/lgtm

[Juneezee]

/retest