Captain reported executing a go around while on final approach after an aircraft taxied onto the runway.

Narrative

On ILS 26 approach we picked up the airport and cancelled IFR about 3 miles out. We made radio calls during the entire approach. At less than 200 ft. and 1/4 mile from touchdown Aircraft X made a radio call 'taking the Runway' and started pulling out onto the Runway. I immediately called traffic hold; traffic hold but they continued and never made another call on RIL CTAF. Our options were limited since the ceilings were low; we were in a valley and Aircraft X departure was the same path as the missed approach on ILS 26. To avoid collision and loosing sight of the aircraft. By going back into the clouds we opted for a low right circle to land. After calling a Right 360 back to Runway 26 I switched back to Denver Center 134.95. I asked them to inform the aircraft. On approach behind us of the circle to avoid collision. At that time they queried Aircraft X. Their response was 'our nose was already out on the Runway; We didn't see them until the last moment'. We returned safely to RIL but the outcome could have easily ended in disaster. I made contact with the company safety department who said that he would get a [report] for from their crew but I have heard nothing back from them. I also have the audio recording of RIL CTAF of the minutes the incident transpired. RIL Airport Ops is getting the video of the incident and has informed us that this is not the only time this has happened this season.