pyrex.ini: add /tmp to default [run]bind

I observed anomalously high disk I/O on my root filesystem during an
OpenEmbedded build which used pyrex. (In fact, *no* rootfs I/O should generally
be happening on this machine during an OpenEmbedded build; both the sources and
TMPDIR are on other disks, and `/tmp` is a tmpfs.) After some judicious debugging
with fatrace(1) I noticed that virtually all of the I/O was happening against,
e.g.,
`/var/lib/docker/overlay2/dc702bbbb061fbbf13b9d06c36d8ccee398257070e6c0455ff209b656b2db2f3/diff/tmp/ccuwDoBf.o`.

It appears that under pyrex, bitbake will use the `/tmp` in the container
overlay, not the system's `/tmp`. I believe that this is a mistake: pyrex
containers should *always* use `/tmp` directly through a bind mount. My
reasoning is as follows.

- On systems where `/tmp` is a tmpfs, using it will effect a major performance
  improvement, and potentially reduce root filesystem wear.
- By using a tmpfs instead of the root filesystem, this change cannot introduce
  any OOM that wouldn't have happened anyway without pyrex.
- The use of `/tmp` in recipes must already be robust in the presence of
  parallel builds; any collision between simultaneous builds under different
  containers represents a fault in the recipe, not in the bind-mounting.
- No security problems can occur through bind-mounting `/tmp` that would not
  have happened anyway when building outside of pyrex.

pyrex.ini

@@ -94,6 +94,7 @@ confversion = @CONFVERSION@
 # A list of directories that should be bound when running in the container
 %bind =
 %   ${env:PYREX_BIND}
+%   /tmp
 
 # A list of environment variables that should be propagated to the container
 # if set in the parent environment