test(pkg/config): use t.Setenv to set env vars

[Juneezee]

What is the problem I am trying to address?

Ensure no leftover environment variables between tests in pkg/config/config_test.go.

How is the fix applied?

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.

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

func TestFoo(t *testing.T) {
	// before
	oldVal := os.Getenv("key")
	defer os.Setenv("key", oldVal)
	os.Setenv(key, "new value")
	
	// after
	t.Setenv(key, "new value")
}

What GitHub issue(s) does this PR fix or close?