docs: Document how to replicate a CI build locally.

Who hasn't needed to do this at some point?  Turns out it's not too hard
to do, and was useful for me in iterating on the Android build.

Acked-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6700>
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