cmd/go: test tmpdir failure

[gopherbot]

#!watchflakes
post <- pkg == "cmd/go" && test == "" && `unexpected files left in tmpdir` && `vcstest\d+`

Bug automatically created to track these flakes.

— watchflakes

[gopherbot]

Found new matching flaky dashboard failures for:

#!watchflakes
post <- pkg == "cmd/go" && test == "" && !`panic: test timed out`

2022-08-16 01:54 windows-amd64-2012 go@5e7697b6 cmd/go (log)

go test proxy running at GOPROXY=http://127.0.0.1:51191/mod
PASS
2022/08/16 02:13:18 unexpected files left in tmpdir: [go-build2656576543]

— watchflakes

[bcmills]

Duplicate of #30789

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- pkg == "cmd/go" && test == "" && `unexpected files left in tmpdir`

2023-03-22 17:41 linux-amd64-longtest go@b52a6963 cmd/go (log)

vcs-test.golang.org rerouted to http://127.0.0.1:34315
https://vcs-test.golang.org rerouted to https://127.0.0.1:34747
go test proxy running at GOPROXY=http://127.0.0.1:39473/mod
2023/03/22 18:11:43 http: TLS handshake error from 127.0.0.1:45120: read tcp 127.0.0.1:34747->127.0.0.1:45120: read: connection reset by peer
2023/03/22 18:11:43 http: TLS handshake error from 127.0.0.1:45146: EOF
2023/03/22 18:11:44 http: TLS handshake error from 127.0.0.1:45170: read tcp 127.0.0.1:34747->127.0.0.1:45170: read: connection reset by peer
2023/03/22 18:11:44 http: TLS handshake error from 127.0.0.1:45190: EOF
2023/03/22 18:11:44 http: TLS handshake error from 127.0.0.1:45196: read tcp 127.0.0.1:34747->127.0.0.1:45196: read: connection reset by peer
2023/03/22 18:11:44 http: TLS handshake error from 127.0.0.1:45212: EOF
2023/03/22 18:11:44 http: TLS handshake error from 127.0.0.1:45228: EOF
2023/03/22 18:12:08 http: TLS handshake error from 127.0.0.1:45852: EOF
FAIL
2023/03/22 18:14:03 unexpected files left in tmpdir: [vcstest1984193402]

— watchflakes

[bcmills]

That directory is created here:
https://cs.opensource.google/go/go/+/master:src/cmd/go/internal/vcweb/vcstest/vcstest.go;l=49-57;drc=8749d12bb76db77cdf37d47d666a5962b8a5928b