Nexus 5 has no settings to change runtime
From the official Android Lollipop website,

Android 5.0 provides a faster, smoother and more powerful computing experience.
Android now runs exclusively on the new ART runtime, built from the ground up to support a mix of ahead-of-time (AOT), just-in-time (JIT), and interpreted code. It’s supported on ARM, x86, and MIPS architectures and is fully 64-bit compatible.

(Emphasis mine)
So, starting from Android 5.0 Lollipop, it will run on Android Runtime (ART). AFAIK, there is no way to change back to Dalvik runtime.