help2man: Use GNU_MIRROR environment variable

Author: reet-

recipes/devel/help2man.yaml

@@ -4,7 +4,7 @@ metaEnvironment:
 
 checkoutSCM:
     scm: url
-    url: "https://ftp.gnu.org/gnu/help2man/help2man-${PKG_VERSION}.tar.xz"
+    url: ${GNU_MIRROR}/help2man/help2man-${PKG_VERSION}.tar.xz
     digestSHA1: "3ed88430c97af3c5b57949f6f030b913044af507"
     stripComponents: 1