Thank you.
Einstein didn't measure the speed of light. He wasn't an experimental physicist. He was a theoretical physicist. Theoretical physicists don't make contraptions to measure things.
The actual velocity of one particular guage boson in free space is a happy accident and a convenient way to measure c, but “c” is much deeper than that. It’s a structural constant of spacetime, and whatever its value happens to be, spacetime will behave as the appropriate field equations require with that value of “c” subsituted in for the lorentz tranformations etc. What actual photons do is largely irrelevant since einstein’s field equations would still be true if there was no charge and hence no EM fields to give rise to photons in the first place.