Use the parent's Subscription when checking for Specialized SIG

[JenGoldstrich]

In v1.4.4 of the plugin a regression was shipped (#321) where when checking if a source SIG image is specialized or not we were passing in the build's subscription, because of this if you built a SIG image based on a SIG image in another subscription in v1.4.4 you receive the following error

==> azure-arm.linux-sig: Running builder ...
==> azure-arm.linux-sig: Getting tokens using Azure CLI
==> azure-arm.linux-sig: Getting tokens using Azure CLI
    azure-arm.linux-sig: Creating Azure Resource Manager (ARM) client ...
Build 'azure-arm.linux-sig' errored after 1 second 97 milliseconds: the parent Shared Gallery Image 'acctestgallery' from which to source the managed image version to does not exist in the resource group 'jenna-other-sub' or does not contain managed image 'arm-linux-specialized-sig'

==> Wait completed after 1 second 97 milliseconds

==> Some builds didn't complete successfully and had errors:
--> azure-arm.linux-sig: the parent Shared Gallery Image 'acctestgallery' from which to source the managed image version to does not exist in the resource group 'jenna-other-sub' or does not contain managed image 'arm-linux-specialized-sig'

==> Builds finished but no artifacts were created.

After this change trying to build the same image is successful.

Closes #321

[nywilken]

LGTM

[shyoon-devops]

Cool!! :)