rust: doc: Zip up docs when done

Fetching artifacts from the build server doesn't go well with large
numbers of files in nested directories.

Produce an additional rustdoc.zip artifact to make this easier.

This change also prevents ${DIST} from receiving the unpacked rustdoc
site. This can be changed if b/188822051 is fixed.

Bug: 162741284
Test: m rustdoc; check resulting zip contents manually
Change-Id: I80b6a8fa6e274d2d8c3419d8734251afd4d7dba7