_sandboxreapi.py: Set platform properties

This sets platform properties for buildbox-run and remote execution.

• Canonicalize OS name to match REAPI platform lexicon (lower case and use macos on macOS). This results in a cache key change.
• Set OSFamily and ISA platform properties for both buildbox-run and remote execution. This ensures the action is handed to a suitable worker and allows buildbox-run implementations to support non-native compilation (as well as linux32 handling).
• Set unixUID and unixGID for buildbox-run to support build-uid and build-gid from project.conf and .bst files. See BuildGrid/buildbox/buildbox-run-bubblewrap!8 (merged). This will allow tests/integration/build-uid.py to pass with buildbox-run-bubblewrap. However, I'm deferring the drop of the skipif until the next update of buildbox-run-bubblewrap in the docker images used in CI.
• Set network for buildbox-runto support network access when invokingbst shellwithout--isolate`. Depends on BuildGrid/buildbox/buildbox-run-bubblewrap!9 (merged)

This is part of #719 (closed).