Allow own bazel remote cache and bazel config

## Enhancement
The BAZEL_CMD_CONFIG is always set in Makefile.common, since it uses `:=` while if it was `?=` instead, it would only be set if not already set.

Open Makefile.common and inspect the definition of BAZEL_CMD_CONFIG, focusing on how its assignment handles an existing environment value. Change the assignment behavior so a user-provided value is preserved, then verify that an unset value still receives the project default.

Written by the indexing model from the issue text.