make execution-policy=as-nobody the default on Linux

[dws]

I naively tried using a buildfarm 2.10.0 started via

./examples/bf-run start

per https://bazelbuild.github.io/bazel-buildfarm/docs/quick_start/#container-quick-start in conjunction with a repo where Bazel had

build --remote_default_exec_properties=execution-policy=as-nobody

in .bazelrc. When I used --remote_executor=grpc://localhost:8980 to point Bazel at the local buldfarm, all actions appeared to get stuck (I gave up after five minutes).

When I dropped the --remote_default_exec_properties setting, the local buildfarm was able to process remote executions.

The behavior was the same with either Bazel 6.2.0 or 7.1.1 as the client.

I don't particularly mind needing to drop the exec properties, but it would have been nice to get some sort of diagnostic somewhere to finger that as the problem.

Alternatively, perhaps as-nobody could become the default buildfarm behavior on Linux? There would presumably need to be a way to turn that off for repos that were not ready for it yet.