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

[Juneezee]

Issue #, if available:

Description of changes:
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. This ensures that each test does not start with leftover environment variables from previous completed tests.

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")
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[snay2]

LGTM. Thank you for the contribution!

[cjerad]

Included in release v1.18.2