Make CIFuzz building accept an env var `MANUAL_SRC_PATH` that points to a manually prepared checkout of the project-under-tests's source code. This allows projects like Skia which are not on OSS-Fuzz and/or need to make changes to the repo after checking out the right commit to use CIFuzz.
Note that for now we aren't supporting this in GitHub, so projects that need to modify the source after checkout can't use it until
we do support it.
Also, use the local copy of OSS-Fuzz when building cifuzz-base instead of cloning it from GitHub. This makes local debugging/development much easier since it allows one to use CIFuzz with local changes.
jonathanmetzman