Add cache-only mode using `LocalExecutionClient`
This depends on BuildGrid/buildbox/buildbox-common!523.
New buildbox-e2e test is in BuildGrid/buildbox/buildbox-e2e!72 (merged)
Edited by Jürg Billeter
This depends on BuildGrid/buildbox/buildbox-common!523.
New buildbox-e2e test is in BuildGrid/buildbox/buildbox-e2e!72 (merged)