Tests: use t.Setenv to set env vars (#69516)

This commit replaces `os.Setenv` with `t.Setenv` in tests. 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

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

pkg/setting/expanders_test.go

@@ -14,8 +14,7 @@ import (
 func TestExpandVar_EnvSuccessful(t *testing.T) {
 	const key = "GF_TEST_SETTING_EXPANDER_ENV"
 	const expected = "aurora borealis"
-	err := os.Setenv(key, expected)
-	require.NoError(t, err)
+	t.Setenv(key, expected)
 
 	// expanded format
 	{