Azure CI: Be explicit about pytest cache directory

The default pytest cache directory is in a read-only directory in Azure,
which results in a warning on the build page. Use the pytest command
line option to set the cache dir to somewhere writable.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/.azure-pipelines.yml b/.azure-pipelines.yml
index ce291d6..5594a67 100644
--- a/.azure-pipelines.yml
+++ b/.azure-pipelines.yml
@@ -430,7 +430,7 @@
           export PATH=/opt/qemu/bin:/tmp/uboot-test-hooks/bin:${PATH};
           export PYTHONPATH=/tmp/uboot-test-hooks/py/travis-ci;
           # "${var:+"-k $var"}" expands to "" if $var is empty, "-k $var" if not
-          ./test/py/test.py -ra --bd ${TEST_PY_BD} ${TEST_PY_ID} ${TEST_PY_TEST_SPEC:+"-k ${TEST_PY_TEST_SPEC}"} --build-dir "$UBOOT_TRAVIS_BUILD_DIR" --report-dir "$UBOOT_TRAVIS_BUILD_DIR";
+          ./test/py/test.py -ra -o cache_dir="$UBOOT_TRAVIS_BUILD_DIR"/.pytest_cache --bd ${TEST_PY_BD} ${TEST_PY_ID} ${TEST_PY_TEST_SPEC:+"-k ${TEST_PY_TEST_SPEC}"} --build-dir "$UBOOT_TRAVIS_BUILD_DIR" --report-dir "$UBOOT_TRAVIS_BUILD_DIR";
           # the below corresponds to .gitlab-ci.yml "after_script"
           rm -rf /tmp/uboot-test-hooks /tmp/venv
           EOF