psub: Use explicit command

This would have prevented #6323.

While we don't want to pepper `command` everywhere, `psub` is kind of
a core thing, so we should try to proof it against common problems.

share/functions/psub.fish

@@ -28,18 +28,18 @@ function psub --description "Read from stdin into a file and output the filename
         set dirname (mktemp -d $tmpdir/.psub.XXXXXXXXXX)
         or return
         set filename $dirname/psub.fifo"$_flag_suffix"
-        mkfifo $filename
+        command mkfifo $filename
         # Note that if we were to do the obvious `cat >$filename &`, we would deadlock
         # because $filename may be opened before the fork. Use tee to ensure it is opened
         # after the fork.
-        tee $filename >/dev/null &
+        command tee $filename >/dev/null &
     else if test -z "$_flag_suffix"
         set filename (mktemp $tmpdir/.psub.XXXXXXXXXX)
-        cat >$filename
+        command cat >$filename
     else
         set dirname (mktemp -d $tmpdir/.psub.XXXXXXXXXX)
         set filename "$dirname/psub$_flag_suffix"
-        cat >$filename
+        command cat >$filename
     end
 
     # Write filename to stdout