ci: avoid "No space left on device" error

Remove unneeded files from the root filesystem to avoid a "No space
left on device" error. A similar trick was committed to optee_os for
the same reason [1].

Link: OP-TEE/optee_os@fa19500 [1]
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@foss.st.com>

.github/workflows/ci.yml

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