integration_tests: don't swallow error while creating ubuntu docker image

[brandsimon]

When there is an error fetching the script and the result is empty, bash does not complain.
The result is a broken image.

[wiktor-k]

I wonder why we aren't using rust official images in the first place but a 5 year old thing 🤔

But that's a separate matter 😅

Thanks for fixing!

[brandsimon]

I think we use the tpm2-software/ubuntu-20.04 image, because it is more difficult to install the tpm2 stuff then it is to install the rust stuff. (Also there are a lot of manual tss2 setup commands)