ci: rust: fix "no space left in device" error

Remove the content of /__t before starting the QEMUv8_check_rust job.
This directory contains things we do not need:

 # du -s /__t/* | sort -n -r | head -n 10
 8651892	/__t/CodeQL
 1246592	/__t/Python
 1015800	/__t/go
 668520	/__t/PyPy
 560996	/__t/node
 63188	/__t/Ruby
 16	/__t/Java_Temurin-Hotspot_jdk

Deleting these files saves 11G of disk space in the root directory:

 $ diff -u df-h_before df-h_after
 [...]
  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

This fixes the "no space left on device" errors that appeared recently
when running the job.

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

.github/workflows/ci.yml

@@ -402,6 +402,8 @@ jobs:
     runs-on: ubuntu-latest
     container: jforissier/optee_os_ci:qemuv8_check2
     steps:
+      - name: Remove /__t/*
+        run: rm -rf /__t/*
       - name: Restore build cache
         uses: actions/cache@v3
         with: