[sancov] fix coverage-report-server cannot display coverage detail

This patch make following change for coverage-report-server.py
- using uri `./{name}` from root in the old version python http.server can be handled as `//{name}`. But due to https://github.com/python/cpython/pull/93879, it will be handled as `/{name}` now.

So I want to use a prefix to avoid double slashes issue.

Differential Revision: https://reviews.llvm.org/D146010