AtTiny analog read always 1023
In this article the following steps are used when using an LED as a light detector:


Light up the LED by making the anode positive (logic one) and the
cathode negative (logic zero) - this fully discharges the LED as a
capacitor.
Charge up the capacitor by making the anode negative and the cathode
positive, as this is a small capacitor it will take no time at all
in other words for as short a time as you can.
Put the diode into the measurement mode, by making the anode
negative (it already is) and the cathode connected to an analogue
input.
Make a measurement of the analogue voltage, this is a reference
level to be used later.
Wait while the photon current is integrated - the longer the more
sensitive but too long and effects other than photo current
dominate.
Measure the voltage again and subtract it from the reference value
you took before the integration time. This figure is your light
reading.


Looking at the code in the above question it is not apparent where you switch from forward bias to revers bias.  Also, it does not appear there are two samples taken and the difference between the two samples is found.