Pass along local resource fraction to reproxy

Reproxy should be respecting the -j value set by the user and not be
doing excessive local executions assuming the whole machine is available
to it. The local_resource_fraction flag is a way to ensure reproxy
consumes CPU only upto the -j value specified by the user.

Test:
1. Ran with `USE_RBE=true m -j 10 nothing` and local_resource_fraction
was set to 0.14 on a 72-core workstation.
2. Ran with `USE_RBE=true m nothing` and local_resource_fraction was set
to 1.0 on a 72-core machine.

Bug: b/207296459
Change-Id: I118bc10109f5a55df7a6c1ecd79499320055f2ed