A Tower Controller working all positions in the Tower including Controller in Charge reported while answering the commercial phone at the supervisor desk they taxied an aircraft on to a runway with another aircraft on short final.

Narrative

I was working Ground and Local combined in the tower on Ground position. I had approved Aircraft X to land the secondary runway not in use from the radar controller. Aircraft X checked on approximately 15 miles from the field and I cleared him to land the secondary RWY XX and used my memory aid turning the data block blue from the local control position. The aircraft was slow. While also working CIC (Controller in Charge) I took a sick leave call and was calling the overtime list attempting to get a full shift to cover. During this time I was also issuing clearances and inputting TMU (Traffic Management Unit) delays into our IDS4 (Information Display System 4). Aircraft Y called for clearance; which I issued. He was positioned east of RWY XX. While I was leaving a message for OT (Overtime) Aircraft Y called ready to taxi. I issued him instructions to taxi to the main runway XY via RWY XX and taxiway 1. I did not catch Aircraft X on short final for RWY XX in my scan from the Ground Control Position.Aircraft Y started his taxi; crossing the hold short line; and at some point said; 'Am I cleared; there's someone landing.' I responded and told him to hold his position as he was just over the hold short line. Aircraft X was at taxi speed on RWY XX when he passed in front of Aircraft Y at RWY XX and TWY 2 intersection. Aircraft X was given exit instructions at TWY 2 and Aircraft Y continued his taxi via RWY XX.I was working combined positions after a two week period being away from the facility on other duties. Specifically working Ground control because I needed currency time for the month. I likely should not have been working the entire tower alone based on those circumstances.My scan was insufficient and I was distracted with Watch Supervisor duties; and working two positions combined. The traffic volume was low; but my work load was higher than I should have been working alone in light of not working traffic for over two weeks.