linux_amd64_longtest flaky: dependency on website(s) for vcs tests

[dagood]

This test hit issues accessing https://vcs-test.golang.org:

https://dev.azure.com/dnceng/internal/_build/results?buildId=1451186&view=ms.vss-test-web.build-test-results-tab&runId=41765234&resultId=101201&paneView=debug

        go: module vcs-test.golang.org/git/prefixtagtests.git/sub: git ls-remote -q origin in $WORK/gopath/pkg/mod/cache/vcs/e451b59ee62e623cdb2396e820b4c1bc9a211927958999ea4f0cc7a47ed1ab04: exit status 128:
        	fatal: unable to connect to vcs-test.golang.org:
        	vcs-test.golang.org[0: 35.184.38.56]: errno=Connection timed out

Earlier instance: https://dev.azure.com/dnceng/internal/_build/results?buildId=1401509&view=ms.vss-test-web.build-test-results-tab&runId=40692980&resultId=101972&paneView=debug

Related upstream issues:

• cmd/go, cmd/go/internal/modfetch: update tests not to rely on external domains that only an individual maintainer has access to golang/go#40519
• cmd/go: migrate tests from vcs-test.golang.org to a test-local server golang/go#27494