Build Tor 0.2.6.6 with seccomp

In feature/8925-tor-0.2.6, we’re using Tor 0.2.6 from the Tor Project’s experimental repository. Before merging, we should instead build our own Tor, with seccomp enabled.

Feature Branch: feature/8925-tor-0.2.6

Parent Task: #7934 (closed)

Original created by @intrigeri on 9147 (Redmine)

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