I wear some tight boxer briefs (cotton and some nylon) from Hugo Boss ($20 for a pack of three) when I go on long rides. Tight enough and dissipates heat fast enough. Not scratchy at all.

Your work jeans will get worn out first before your underwear will. My "fashion" jeans (aka thin denim) got worn out in 6 months of regular riding, so your work jeans should last considerably longer.