How to switch PIR sensor on/off based on photocell result
There are multiple faults with your circuit:


There is insufficient voltage to drive your PIR motion detectors. Reference to the link you provided, the HC-SR505 require from 4.5-20V. You LiPo battery will only provide 4.2 when fully charged, and for most of the discharge cycle will be below 4V. No matter what you use to switch the sensors on/off with, they are unlikely to provide consistent results.
Using a BJT to try and cycle the VCC for the HC-SR505 will reduce the voltage to an even more critical level.
The HC-SR505 much like its bigger brother the HC-SR501 takes a long time to initialize and is VERY sensitive to VCC fluctuations. During initialization time you may see an active output. I've used both and typically wait at about 4-5 seconds after applying power to allow the output to settle. At the VCC voltage you are operating the sensor you are likely to see multiple resets. 
The best advice seems to be that the HC-SR505 uses an EG4001 chip while the HC-SR501 uses a BIS0001, both chips used in the sensors draw low current. You have already found this to be around 100uA. You can switch the VCC to the sensor using any GPIO port on the ATTiny85 as they will handle many mA (though you need the VCC to be high enough for the regulator to work). I've used both the HC-SR505 and 501 on a 5V Arduino driven directly from the GPIO pins and they work fine. You just have to allow initialization before believing the output signal.


A good review of the HC-505 along with a schematic is here.  



You will notice that the input power connection has a series diode, which accounts for the minimum supply voltage of 4.5V.    

Suggestions:  


You could short out the protection diode and the regulator (HT7133) would then work down to about 3.45V which would then work for your LiPo battery.
Connect your sensors directly to the ATTiny85 GPIO pin (both sensors could go to one pin), but ensure you give sufficient time for the sensors to settle when you power them up.  
Connect your ambient light detector (CDS cell) to an analog input with the pullup resistor driven by another GPIO pin. Then when you put the ATTiny to sleep you reduce (switch off) that current.   
Switch off your peripherals and put the MCU to sleep for the longest time you can count. When the MCU wakes up test the ambient light detector. If it's dark you can power up your PIR detectors.   
When your PIR detectors are on, minimize the MCU current by turning off as much as you can (like the A/D and BOD) and use IDLE state with wake on PIR detection.