[WIP] ci: rust: investigate "no space left on device" error

Remove the content of /__t before starting the job. This directory
contains things we do not depend on (tools?):

This saves 11G of disk space:

$ diff -u df-h_before df-h_after
--- df-h_before 2023-10-10 12:13:55.016599574 +0200
+++ df-h_after  2023-10-10 12:14:05.924640985 +0200
@@ -1,8 +1,7 @@
-Filesystem      Size  Used Avail Use% Mounted on
-overlay          84G   67G   17G  81% /
+overlay          84G   56G   28G  67% /
 tmpfs            64M     0   64M   0% /dev
 shm              64M     0   64M   0% /dev/shm
-/dev/root        84G   67G   17G  81% /__w
+/dev/root        84G   56G   28G  67% /__w
 tmpfs           1.4G  1.2M  1.4G   1% /run/docker.sock
 tmpfs           3.4G     0  3.4G   0% /proc/acpi
 tmpfs           3.4G     0  3.4G   0% /proc/scsi

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>