fix var style for powershell

Author: TimHess

.github/workflows/package.yml

@@ -170,8 +170,8 @@ jobs:
         --base-directory '${{ github.workspace }}/packages'
         --azure-key-vault-managed-identity true
         --azure-credential-type 'azure-cli'
-        --azure-key-vault-url "$AZURE_KEY_VAULT_URL"
-        --azure-key-vault-certificate "$AZURE_SIGN_CERTIFICATE_ID"
+        --azure-key-vault-url "$env:AZURE_KEY_VAULT_URL"
+        --azure-key-vault-certificate "$env:AZURE_SIGN_CERTIFICATE_ID"
         --publisher-name 'Steeltoe'
         --description 'Steeltoe'
         --description-url 'https://steeltoe.io/'