progress: fix leak of pipe goroutine from MultiReader

In the case where the context provided to MultiReader.Reader is
cancelled, the writer will be deleted.

Before this, closeWriter was not called in that codepath, when meant
that MultiReader.handle would never see the writer and never close it.
* I don't have an isolated repro, but when running a few hundred of
  Dagger's tests I was seeing several thousand goroutines blocked on
  waiting for the progress pipe's context to be done.

Now, we just call closeWriter when it gets deleted so that the pipe's
goroutine doesn't leak.
* After this change, those thousands of leaked goroutines are entirely
  gone.

Signed-off-by: Erik Sipsma <[email protected]>

util/progress/multireader.go

@@ -95,6 +95,7 @@ func (mr *MultiReader) Reader(ctx context.Context) Reader {
 		mr.mu.Lock()
 		defer mr.mu.Unlock()
 		delete(mr.writers, w)
+		closeWriter(context.Cause(ctx))
 	}()
 
 	if !mr.initialized {