avoid toolchain failure for cross-platform builds

[oliverlee]

When building/testing everything (//...) for a target platform that differs from the host platform, I get the following error:

While resolving toolchains for target //tools:compile_commands (9b8c871): No matching toolchains found for types:
  @@bazel_tools//tools/python:toolchain_type

Would it make sense to set target_compatible_with = HOST_CONSTRAINTS by default? Or to set tags = ["manual"], given that this target needs to be run with bazel run?