Update docs regarding OS environment variables to include MacOS

[jacobq]

In the "Running Windows and Linux builds side-by-side" section of Getting started with AppVeyor for Linux it mentions that in PowerShell we have the following:

$isLinux is $true on Linux
$isWindows is $true on Windows

and in general:

AppVeyor also introduces two new environment variables defining platform:

CI_WINDOWS is true if the build is running on Windows-based image; otherwise false.
CI_LINUX is true if the build is running on Linux-based image; otherwise false.

However, there is no mention of corresponding $isMacOS or CI_MACOS variables. Furthermore, it sounds like $isWindows may not work on Windows (see appveyor/ci#3214 (comment)).

[jacobq]

FWIW, on the macos image I see the following:

worker-628-001:my-project appveyor$ pwsh -c Get-Variable

Name                           Value
----                           -----
...
IsCoreCLR                      True
IsLinux                        False
IsMacOS                        True
IsWindows                      False
...

and

worker-628-001:my-project appveyor$ env
...
CI_MACOS=true
...
CI=true
...
CI_FREEBSD=false
...
CI_WINDOWS=false
...

[jacobq]

It would be nice if the docs also clarified which variables are added by AppVeyor vs. being present already, e.g. https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_automatic_variables?view=powershell-7#islinux