No /system/build.prop in Nexus 7 (2012 / Wi-Fi) + Android Lollipop 5.1.1 after fresh installation
The /system partition wasn't mounted; I've noticed this only manually browsing the /system/bin/ folder, which was empty.

I mounted the partition within the recovery mode by browsing to "mounts and storage" and by selecting "mount /system"; afterwards I've been able to run adb pull /system/build.prop ., edit the file and upload it again running adb push build.prop /system/build.prop.

To unmount the partition afterwards, one might browse to "mounts and storage" and select "unmount /system".