crypto/tls: use ChaCha8 source during benchmarks #81344

Follow up to CL 780102 for the Go 1.28 cycle, in the form of an issue so it can be tracked.

I mailed CL 780102 in response to a break in the crypto/tls benchmarks (#79468, #79521, and #79522) late in the Go 1.27 cycle. That CL used all zeroes for the entropy source for those benchmarks, which worked for the Go 1.27 release but isn't a good long-term strategy. They should use a Reader that doesn't give a trivial pattern; ChaCha8 with a fixed seed would work. CL 779984 implements that.

CC @golang/security @FiloSottile @thatnealpatel