Fixes https://github.com/hashicorp/nomad/issues/5593

Executor seems to die unexpectedly after nomad agent dies or is
restarted.  The crash seems to occur at the first log message after
the nomad agent dies.

To ease debugging we forward executor log messages to executor.log as
well as to Stderr.  `go-plugin` sets up plugins with Stderr pointing to
a pipe being read by plugin client, the nomad agent in our case[1].
When the nomad agent dies, the pipe is closed, and any subsequent
executor logs fail with ErrClosedPipe and SIGPIPE signal.  SIGPIPE
results into executor process dying.

I considered adding a handler to ignore SIGPIPE, but hc-log library
currently panics when logging write operation fails[2]

This we opt to revert to v0.8 behavior of exclusively writing logs to
executor.log, while we investigate alternative options.

[1] https://github.com/hashicorp/nomad/blob/v0.9.0/vendor/github.com/hashicorp/go-plugin/client.go#L528-L535
[2] https://github.com/hashicorp/nomad/blob/v0.9.0/vendor/github.com/hashicorp/go-hclog/int.go#L320-L323