--remote_{downloader,cache,executor} all support reading credentials from .netrc: https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/remote/RemoteModule.java;l=1098;drc=1bc1ce3e0d73c35047c29ef85e832db041e0f238

--bes_backend doesn't: https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/buildeventservice/BazelBuildEventServiceModule.java;l=79;drc=4cd266aa1dfa53d8c8de44f7895edcfd46f74725 calling into https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/authandtls/GoogleAuthUtils.java;l=194;drc=4cd266aa1dfa53d8c8de44f7895edcfd46f74725

Found while working on #15856