ci: hafnium: fix "no space left on device" error

Apply the same technique as commit 788069f ("ci: rust: fix "no space
left in device" error") to avoid a similar issue with the Hafnium job.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@foss.st.com>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>

.github/workflows/ci.yml

@@ -330,6 +330,8 @@ 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: