dist,unix: use MKSHELL=$PLAN9/bin/rc (John DeGood)

dist/mkfile

diff --git a/dist/mkfile b/dist/mkfile
index 28cf31c..06bdbb3 100644
--- a/dist/mkfile
+++ b/dist/mkfile
@@ -1,4 +1,4 @@
-MKSHELL=rc
+MKSHELL=$PLAN9/bin/rc
 
 check.out:V:
 	cd ../man; mk indices

unix/mkfile

diff --git a/unix/mkfile b/unix/mkfile
index 5d5983b..3d9d35a 100644
--- a/unix/mkfile
+++ b/unix/mkfile
@@ -1,4 +1,4 @@
-MKSHELL=rc
+MKSHELL=$PLAN9/bin/rc
 new-%:V:
 	t=$stem
 	if(! ~ $t utf fmt bio regexp mk){