x11/boomer: attempt to unbreak the port in the build-as-user mode

Build artifacts are created under ~/.cache/nim/$projectname(_r|_d)
by default and this naturally fails because nobody's home directory
is /nonexistent.  Use ${WRKDIR}/.nimcache instead which definitely
exists and is writable.

Nagged by:	pkg-fallout

x11/boomer/Makefile

@@ -32,7 +32,8 @@ post-patch:
 
 do-build:
 	cd ${BUILD_WRKSRC} && nimble build --nim=${LOCALBASE}/nim/bin/nim \
-	--localdeps --offline --reject --verbose -d:release -d:nimDebugDlOpen
+	--nimcache:${WRKDIR}/.nimcache --localdeps --offline --reject \
+	--verbose -d:release -d:nimDebugDlOpen
 
 do-install:
 	${INSTALL_PROGRAM} ${INSTALL_WRKSRC}/${PORTNAME} \