Wait to finish reading logs before calling Wait() on pipeline

[thestr4ng3r]

This fixes errors like the following and chopped off
logs with the local backend:

{"level":"error","error":"read |0: file already closed","time":"2022-07-04T17:42:29+02:00","message":"copy limited logStream part"}

The reason for this issue is that exec's Wait(), which is used
internally by the this backend, closes the stdout/stderr pipe, so we
must wait until all reading is done before calling it.

See https://pkg.go.dev/os/exec?utm_source=gopls#Cmd.StdoutPipe:

Wait will close the pipe after seeing the command exit, so most
callers need not close the pipe themselves. It is thus incorrect
to call Wait before all reads from the pipe have completed.

Instead of using a WaitGroup, one could call the reading synchronously,
but with the step.Detached case probably still needing it as a goroutine,
it might become less readable.

An alternative solution could be to move this synchronization into the local backend
implementation itself, for example by constructing a new io.Pipe(), copying everything
into it and waiting for this to be done before calling the exec Wait().
But I think it might be better in general to wait until all log has been read before considering
a pipeline finished.

I have also sometimes encountered errors like these:

{"level":"error","error":"read |0: file already closed","time":"2022-07-04T17:42:29+02:00","message":"copy limited logStream part"}
{"level":"error","error":"rpc error: code = Unknown desc = database is locked","time":"2022-07-04T17:42:29+02:00","message":"grpc error: upload(): code: Unknown: rpc error: code = Unknown desc = database is locked"}
{"level":"error","repo":"thestr4ng3r/ci-test","build":"23","id":"178","image":"/bin/bash","stage":"build","error":"rpc error: code = Unknown desc = database is locked","time":"2022-07-04T17:42:29+02:00","message":"log stream upload error"}

Maybe it is related. In those cases, I did not get any logs at all in the webui. After the fix, I have not seen those anymore, but they were sporadic before too.