pyrex.ini: bind-mount /tmp to /tmp

[rtollert]

It was observed that the root filesystem on my dev machine had an unusually large amount of I/O during NILRT builds, sometimes sustaining over >50MB/s. This is not expected, because with the exception of the pyrex containers (which one would naively consider to be relatively static beasts), my NILRT build does not use the root filesystem in any way.

Some judicious use of fatrace indicated that the vast majority of the I/O was coming from e.g. /var/lib/docker/overlay2/dc702bbbb061fbbf13b9d06c36d8ccee398257070e6c0455ff209b656b2db2f3/diff/tmp/ccuwDoBf.o. What's going on is that the tasks are using the container /tmp instead of the system /tmp. On many machines, including mine, /tmp is a tmpfs, and writing instead to the container /tmp causes needless wear on the root filesystem.

The fix is to simply always bind-mount the container /tmp to the system /tmp.

This change cannot induce any additional build system instability due to to tmpfs-related OOMs, because to a first order, all of our upstream recipes have to deal with this anyway: presumably, not all Yocto developers use pyrex nowadays.