FR: documentation for which system files are read for cloud detection

### What are you trying to do?

I'm trying to look into restricting access to system files (in relation to developing a strictly confined snap), and trying to determine which files that tailscaled needs read access to for cloud detection (and any other purposes if applicable?).

On startup, tailscaled appears to read:

- /sys/devices/virtual/dmi/id/product_name
- /sys/devices/virtual/dmi/id/bios_vendor

But I can't find reference to these paths in the codebase. I do however see other paths:

https://github.com/tailscale/tailscale/blob/65fe0ba7b50928eff53e2750ff2d583e4a33eb7e/util/cloudenv/cloudenv.go#L115-L126

But these don't appear to be read. Could it be confirmed which files are needed? Thanks.

### How should we solve this?

_No response_

### What is the impact of not solving this?

_No response_

### Anything else?

_No response_