Gitlab CI set different environment variables for windows and mac

Is there a way to specify different environment variables for a runner depending on platform?
I build a win and mac version of my app but I only code sign the mac app. I don’t want to sign the windows app yet if I specify environment variables with CSC_LINK in the CI settings of my project, the windows app thinks I’m specifying a certificate that doesn’t exist.
Is there any way to prevent the window CI file from seeing this CSC_LINK variable?