Use artifacts from bst-1.2 branch

Tom Coldrickrequested to merge
coldtom/1.2.x-docs into master

At present the docs website uses artifacts from the master branch, which is increasingly unhelpful for people using 1.2.x. As this is the recommended stable branch, it makes sense for the documentation to be for this version. This uses the latest artifact from the bst-1.2 branch of BuildStream, which should be fine, but it would be nicer to get it to automatically use artifacts from the last tag on that branch, if that's possible.

Fixes #1 (closed)

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