CI: Github Actions: temporarily disable Opentracing

As discussed in the thread below, the recent variables changes
unfortunately broke Opentracing. Discussions are ongoing about
possible solutions but none of them can be done in a 3-liner so
we'd rather disable opentracing from the full-features build for
the time being.

Link: https://www.mail-archive.com/haproxy@formilux.org/msg41131.html
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