CI: Make use of buildman requirements.txt

Now that buildman has a requirements.txt file we need to make use of it.

Reviewed-by: Simon Glass <sjg@chromium.org>
[n-francis@ti.com: Adding missing command from .azure-pipelines.yml]
Signed-off-by: Neha Malcom Francis <n-francis@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>