add jasmin installation to external CI jobs

[fdupress]

Draft PR to check effect on CI times for now. This is to eventually, ultimately, and finally enable #725.

[fdupress]

Time impact is about 10 seconds per external CI job. Not sure about the bandwidth impact on the image. (Didn't measure before losing access to a convenient machine; we could usefully layer it to keep both images.)

@strub does this deserve more engineering to make sure we only install Jasmin for external CI jobs that request it, or are we happy to pay the small performance price for a simpler CI job?

[fdupress]

Outcomes:

• time cost is acceptable;
• layering on docker image is desirable—add jasmin dependencies as a layer on top of the base image.