cmd/coordinator: stop using http.DefaultServeMux
cmd/coordinator: stop using http.DefaultServeMux

Explicitly define a ServeMux and routes that we want to expose to
prevent dependencies from registering unwanted handlers.

Fixes golang/go#50894

Change-Id: Ic083b346efd6e5ad33dd6d5a45c2a63e5c61aa0e
Reviewed-on: https://go-review.googlesource.com/c/build/+/381737
Trust: Alex Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Run-TryBot: Alex Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/build,golang/build,golang/build,golang/build