cmd/dist: wait for exit in bgwait after a fatal error

Currently, when a command scheduled by bgrun fails, bgwait returns to
the caller even in case the fatal error was detected.

In case of a syntax error in one of the standard packages, as an
example, the runInstall function will try to read the generated archive
file, only to fail since the file does not exist.  Since the runInstall
function is called in a goroutine, cmd/dist will continue to report
errors until all background goroutines are done.

Update the bgwait function to wait until program termination in case of
a fatal error, since returning to the caller (with an error, as an
example) will cause cmd/dist to report an additional error during the
next build phase.

Fixes #45410

Change-Id: If89976abad70f8d6ec79b2a5a1f2306e9c034c5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/320311
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>

src/cmd/dist/util.go

@@ -172,6 +172,9 @@ func bgwait(wg *sync.WaitGroup) {
 	select {
 	case <-done:
 	case <-dying:
+		// Don't return to the caller, to avoid reporting additional errors
+		// to the user.
+		select {}
 	}
 }