Powering 5V external sensors from 3V Coin Cell Embedded System
If you can use two coin cells in series then power Arduino from the middle 3.3V and the sensor from 6.6V. 

Also using an LDO with enable and reset to bring 6.6 to 5V you can put the sensor to sleep using enable input and benefit from a "power good" signal from the reset output. Here is an example: http://www.ti.com/product/TPS779