Why not talk to your colleagues? They may have an unused bike in the garage...or just put an add into the CERN market.
Otherwise, it is possible to rent a bike for free (or at least free for the first...

CERN rents out bicycles (in summer there are long waiting lists!). Ask your departmental  secretary upon arrival or click here.

CERN BIKE SHARING:
Members of CERN community can borrow "velopass" bikes...

Some possibilities are:

• Association pour la Récupération de vélos
• Fourierre Velo
• OLX. You can sign up and place an advert for a bike.There are often people advertising their second-hand bikes for sale.