rcpu: avoid filedescriptor conflict with <{} (thanks mycroftiv)

the rcpu client dup's fd 0,1,2 to fd 10,11,12 which
can accidentally override the pipe file descriptor
allocated by the <{} operator. to avoid this problem,
we generate the remote script as an /env file in a
separate step now.

rc/bin/rcpu

@@ -80,4 +80,9 @@ fn pvar {
 	}
 }
 
-exec $connect $host <{dir=`{pwd} pvar dir cmd; builtin whatis server; echo server} client <[10=0] >[11=1] >[12=2]
+>/env/rscript {
+	dir=`{pwd} pvar dir cmd
+	builtin whatis server
+	echo server
+}
+exec $connect $host /env/rscript client <[10=0] >[11=1] >[12=2]