Allow sharing traces via `MAGIC_TRACE_SHARE_COMMAND`

[Xyene]

Tested with MAGIC_TRACE_SHARE_COMMAND=/usr/bin/echo.

Also fixed a bug where we were unconditionally serving the UI if the Perfetto base dir was specified. We lose port configurability, but it's not a big deal and we can add it back as a separate flag if we want to.

[cgaebel]

Port configurability is a big deal! We all use the same machine at work to do magic-trace stuff. That said, I'm fine with adding it back in a separate feature.

[Xyene]

I'm not sure which I like more:

• a -serve-port flag, or
• starting from 8080 and trying increasing port numbers until we find one we can bind to

[cgaebel]

My heart says (2, but start at 1024), but my brain says (1). S'up to you.