TMPDIR redirection can cause absolute UNIX socket paths to be too long

The env TMPDIR redirection can cause absolute UNIX socket paths to be too long.

e.g.
```
virt-v2v-in-place -i disk disk.qcow2

[ 0.0] Setting up the source: -i disk disk.qcow2
stderr:
qemu-nbd: UNIX socket path '/home/lukas/Dokumente/buck2/buck2/examples/remote_execution/test/buck-out/v2/tmp/root/524f8da68ea2a374/vm/__WinDevEval_VirtIO__/genrule/v2v.xUoDfB/in0' is too long
Path must be less than 108 bytes
virt-v2v-in-place: error: qemu-nbd did not start up. There may be errors printed by qemu-nbd above.
```

For now I am resetting TMPDIR back to /tmp/ from the genrule cmd.

Bazel reverted similar changes because of the socket path length problem. (TMPDIR is set there to /tmp/).