Fix Linux local mode missing traffic from non-main threads

[samatar26]

Fixes mitmproxy/mitmproxy#8282

ctx.pid() is the thread id, not the process id so sockets from non-main
threads bypassed Linux local capture. Changed to ctx.tgid(), matching macOS/Windows.

[jackyzy823]

Passing-by comment:
I found this issue too , but not sure if i understand pid/tgid correct or not.
Then i found that you have already opened a PR !

so user side 's PID is actually kernel side TGID.
Ref:
https://stackoverflow.com/questions/9305992/if-threads-share-the-same-pid-how-can-they-be-identified

[samatar26]

@jackyzy823 yeah exactly - and the behaviour in Linux of mitmproxy is different from macOS and Windows so I believe it must be a bug. Mocking traffic of a python program that's making requests from multiple threads works fine on my macOS machine, but fails on a Linux based GitHub action runner