opensp: display ftp_proxy and http_proxy during build

pkgs/tools/text/sgml/opensp/setup-hook.sh

@@ -9,6 +9,14 @@ addSGMLCatalogs () {
 if test -z "$sgmlHookDone"; then
     sgmlHookDone=1
 
+    # Set http_proxy and ftp_proxy to a invalid host to prevent
+    # xmllint and xsltproc from trying to download DTDs from the
+    # network even when --nonet is not given.  That would be impure.
+    # (Note that .invalid is a reserved domain guaranteed not to
+    # work.)
+    export http_proxy=http://nodtd.invalid/
+    export ftp_proxy=http://nodtd.invalid/
+
     export SGML_CATALOG_FILES
     envHooks+=(addSGMLCatalogs)
 fi