Move Environment Sensor BOOTSEL button to opposite side of PCB

With the BME688 on one side of the PCBA, and the remaining components on the other side, accessing the button once the device has been installed on a mounting point is not possible.

While we don't normally need access to this button once working firmware has been uploaded at least once, moving the button would give us the flexibility to flash the device without removing it from its installation location under any circumstance.