Didn't California recruit prisoners to fight the wildfires?
There could be several reasons for the Russians to be doing so. It's cheap, quick and has relatively low impact politically.
I wouldn't necessarily presume the Russians are running out of manpower.
They're not running out of manpower per se. But the equation is far more complicated.
1. They cannot institute conscription without destabilizing the regime and imperiling the only guy who really counts in Russia. He doesn't want to end up in a shallow grave at the side of a road in Siberia and so he avoids doing stuff that would annoy too many of his subjects.
2. To make troops effective, they must be trained and motivated. Conscripts are neither. At this point, Russia is doing human wave attacks with massive artillery prep and security forces with machine guns "encouraging" the conscripts to charge "valiantly". This nets Russia marginal gains every few weeks, but nothing major. Whether this remains viable, remains to be seen. When those artillery pieces wear out, then it's human wave attacks without artillery prep.....
3. There is less and less equipment to give to the shitty untrained troops.
So the Russian army is increasingly ineffective and there is probably no solution for this.