Prevent mk from ignoring errors

By default, the plan9port INSTALL script doesn't report build error,
which can result in a mostly broken build begin installed.
--- a/INSTALL
+++ b/INSTALL
@@ -41,7 +41,6 @@ echo " "
 echo "* Resetting $PLAN9/config"
 rm -f config
 
-(
 echo "* Compiler version:"
 9c -v 2>&1 | grep -v 'Configured with:' | grep -i version | sed 's/^/	/'
 
@@ -134,7 +133,7 @@ if $dobuild; then
 		echo "* Warning: not all binaries built successfully."
 	fi
 	echo "* Installing everything in $PLAN9/bin..."
-	mk -k install || exit 1
+	mk install || exit 1
 	if [ ! -x $PLAN9/bin/cleanname -o ! -x $PLAN9/bin/acme -o ! -x $PLAN9/bin/sam ]; then
 		echo " "
 		echo "* Warning: not all binaries built successfully."
@@ -182,5 +181,4 @@ if $doinstall; then
 	echo "	PLAN9=$PLAN9 export PLAN9"
 	echo '	PATH=$PATH:$PLAN9/bin export PATH'
 fi
-) 2>&1 | tee install.log | $awk -f $PLAN9/dist/isum.awk -v 'copy='install.sum