Nope. There's nothing safe that would deter a budgie. Accepting that parts of your house may get chewed on is part of being a parrot owner.

The best thing is to redirect them to chew other things like toys and put a physical barrier over what they want to chew.

My recommendations: If your window sill is wide enough, put standing perches on it. Budgies like to perch high so they will prefer the perches over the lower sill. Use smooth rocks like river rocks in the spaces around the perches in case the budgie still chooses the sill and put something like a bird kabob nearby to attract any chewing attention.

You can also tryi buying or making a sill cover like https://www.amazon.com/Clear-Pet-Window-Sill-Protector/dp/B079J6B3FL/ref=pd_lpo_1?pd_rd_i=B079J6B3FL&th=1 or https://www.sillshield.com/white.html