Fix failing unit get_env_fallback

Something something string literals on Windows...

test/test_clinkahead.cpp

@@ -44,8 +44,9 @@ protected:
 };
 
 TEST_F(test_clinkahead, test_get_env_fallback) {
-  const char *const some_var = linkahead_utility_get_env_fallback("SOME_ENV_VAR", "fall-back");
-  EXPECT_EQ("fall-back", some_var);
+  constexpr auto fall_back = "fall-back";
+  const char *const some_var = linkahead_utility_get_env_fallback("SOME_ENV_VAR", fall_back);
+  EXPECT_EQ(fall_back, some_var);
 }
 
 TEST_F(test_clinkahead, test_other_client_error) {