zim writes to HOME

When building zim, it leaves a /tmp/zim-zim-0.73.5_writes_to_HOME/
behind. The build log shows these warnings (among others):

Environment variable $HOME does not point to an existing folder: /zim-0.73.5_writes_to_HOME
...
XDG_RUNTIME_DIR is not set, using /tmp/zim-zim-0.73.5_writes_to_HOME as a fallback

This fixes it. No change in the package.

Index: Makefile
===================================================================
RCS file: /cvs/ports/productivity/zim/Makefile,v
retrieving revision 1.26
diff -u -p -r1.26 Makefile
--- Makefile 29 Apr 2021 22:31:00 -0000 1.26
+++ Makefile 12 May 2021 20:53:02 -0000
@@ -32,4 +32,7 @@ RUN_DEPENDS = ${COMMON_DEPENDS} \
misc/shared-mime-info \
x11/gtk+3,-guic

+PORTHOME = ${WRKDIR}
+MAKE_ENV += XDG_RUNTIME_DIR=${PORTHOME}
+
.include <bsd.port.mk>