core::shadow: fix URL

Use the mirror variable.

Author: jkloetzke

recipes/core/shadow.yaml

@@ -13,7 +13,7 @@ depends:
 
 checkoutSCM:
     scm: url
-    url: https://github.com/shadow-maint/shadow/releases/download/${PKG_VERSION}/shadow-${PKG_VERSION}.tar.xz
+    url: ${GITHUB_MIRROR}/shadow-maint/shadow/releases/download/${PKG_VERSION}/shadow-${PKG_VERSION}.tar.xz
     digestSHA256: bb5f70639a0581f9d626f227ce45b31ac137daa7c451c0f672ce14f2731a96ee
     stripComponents: 1