The MSI installer sets a special sentinel value that we can use to detect it.
I also removed the code that bails out when the installation path is not
`Program Files`, as both the NSIS and MSI installers permit the user to install
to a different path.
Signed-off-by: Aaron Klotz <aaron@tailscale.com>
We replace the cmd.exe invocation with RtlGetNtVersionNumbers for the first
three fields. On Windows 10+, we query for the fourth field which is available
via the registry.
The fourth field is not really documented anywhere; Firefox has been querying
it successfully since Windows 10 was released, so we can be pretty confident in
its longevity at this point.
Fixes https://github.com/tailscale/tailscale/issues/1478
Signed-off-by: Aaron Klotz <aaron@tailscale.com>
