Fixes tailscale/corp#18441
For a few days, IsMacAppStore() has been returning `false` on App Store builds (IPN-macOS target in Xcode).
I regressed this in #11369 by introducing logic to detect the sandbox by checking for the APP_SANDBOX_CONTAINER_ID environment variable. I thought that was a more robust approach instead of checking the name of the executable. However, it appears that on recent macOS versions this environment variable is no longer getting set, so we should go back to the previous logic that checks for the executable path, or HOME containing references to macsys.
This PR also adds additional checks to the logic by also checking XPC_SERVICE_NAME in addition to HOME where possible. That environment variable is set inside the network extension, either macos or macsys and is good to look at if for any reason HOME is not set.
6288c9b41e