Only attempt signing when running on Windows

Checklist

[X] I confirm there are no unresolved issues reported on the Chocolatey Status page.
[X] I have verified this is the correct repository for opening this issue.
[X] I have verified no other issues exist related to my problem.
[X] I have verified this is not an issue for a specific package.
[X] I have verified this issue is not security related.
[X] I confirm I am using official, and not unofficial, or modified, Chocolatey products.

What You Are Seeing?

When attempting to run a build for this PR which is using 0.28.0 of Chocolatey.Cake.Recipe, it is failing, since it is attempting to run Windows PowerShell (via the Cake.PowerShell addin), which is not supported on Linux/Mac.

This problem was masked due to the fact that the signing steps had an additional criteria for the ceritificate to be in place and by running on TeamCity. This criteria didn't apply to the new verify signing task added in 0.28.0, so was removed. However, this then meant it would attempt to run on Linux/Mac when it shouldn't.

We need to be explicit about when these signing related tasks are meant to run.

What is Expected?

Build should not fail.

Tasks should not be executed when they are not meant to be run.

How Did You Get This To Happen?

Attempted to run a build on a PR that is using 0.28.0 of Chocolatey.Cake.Recipe

System Details

N/A

Installed Packages

N/A

Output Log

N/A

Additional Context

N/A

chocolatey / Chocolatey.Cake.Recipe