LibJS/JIT: Only try JIT compilation when LIBJS_JIT is set in environment

Instead of adding a flag to everything everywhere, let's try using an
environment variable this time.

Userland/Libraries/LibJS/JIT/Compiler.cpp

@@ -820,7 +820,7 @@ void Compiler::compile_set_variable(Bytecode::Op::SetVariable const& op)
 
 OwnPtr<NativeExecutable> Compiler::compile(Bytecode::Executable& bytecode_executable)
 {
-    if (getenv("LIBJS_NO_JIT"))
+    if (!getenv("LIBJS_JIT"))
         return nullptr;
 
     Compiler compiler { bytecode_executable };