Flush the debug trace file after each dbg!().
Flush the debug trace file after each dbg!().

When debugging code built with panic=about, the debug trace buffer is
not flushed on a panic and important messages are lost.

This does slow down debug tracing a bit, but this is still better than
unbuffered or line buffered I/O since the dbg!() macro can write out
many lines like a whole function.

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