os: eradicate smallpox after test

Otherwise:

$ go test  -short -cpu=1,1,2,2
--- FAIL: TestLookupEnv (0.00s)
        env_test.go:102: SMALLPOX="virus"
--- FAIL: TestLookupEnv-2 (0.00s)
        env_test.go:102: SMALLPOX="virus"
--- FAIL: TestLookupEnv-2 (0.00s)
        env_test.go:102: SMALLPOX="virus"

Change-Id: Ic1f6dd1bae3c79c4f7da02bc8c30b5e599627a82
Reviewed-on: https://go-review.googlesource.com/9794Reviewed-by: Rob Pike <r@golang.org>

src/os/env_test.go

@@ -101,6 +101,7 @@ func TestLookupEnv(t *testing.T) {
 	if ok || value != "" {
 		t.Fatalf("%s=%q", smallpox, value)
 	}
+	defer Unsetenv(smallpox)
 	err := Setenv(smallpox, "virus")
 	if err != nil {
 		t.Fatalf("failed to release smallpox virus")