fix macOS ARM builds that broke in the latest image update

https://www.appveyor.com/updates/2022/02/27/

Author: cfillion

.appveyor.yml

@@ -92,12 +92,15 @@ for:
       # don't use LibXml2/SQLite3 from Mono
       - sudo rm -r /Library/Frameworks/Mono.framework
 
-      # https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/
-      - sudo curl -k https://curl.se/ca/cacert.pem -o /etc/ssl/cert.pem
-
-      # backport vcpkg bugfixes (TODO: remove after future image updates)
-      - git -C ~/vcpkg fetch
-      - git -C ~/vcpkg restore -s 38d6712d5644ede4ff597e889549f5d540dcf8ff ports/catch2
+      - |-
+        if [ "$appveyor_build_worker_image" = "macos-mojave" ]; then
+          # https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/
+          - sudo curl -k https://curl.se/ca/cacert.pem -o /etc/ssl/cert.pem
+
+          # backport vcpkg bugfixes
+          git -C ~/vcpkg fetch
+          git -C ~/vcpkg restore -s 38d6712d5644ede4ff597e889549f5d540dcf8ff ports/catch2
+        fi
     artifacts:
       - path: build/reaper_reapack*.dylib