cmd/go: potential test flakes for TestScript/mod_invalid_version

[mrkfrmn]

There's a recent failure on the linux/amd64 longtest-aliastypes builder that does not seem related to the commit appended in that build — I suspect it could be a flaky test:

--- FAIL: TestScript (0.19s)
vcs-test.golang.org rerouted to http://127.0.0.1:36369/
https://vcs-test.golang.org/ rerouted to https://127.0.0.1:36743/
go test proxy running at GOPROXY=http://127.0.0.1:46575/mod
--- FAIL: TestScript/mod_invalid_version (166.83s)
    script_test.go:139: 2025-07-30T21:19:41Z
    script_test.go:141: $WORK=/home/swarming/.swarming/w/ir/x/t/cmd-go-test-94143057/tmpdir772596210/mod_invalid_version1703560507
go proxy: no archive example.net/b v0.1.0: file does not exist

Failure is first seen at a4d9977 (AFAICT).

[gabyhelp]

Related Issues

• cmd/go: TestScript/mod_pseudo_cache and TestScript/mod_invalid_version fail #54102 (closed)
• cmd/go: TestScript/mod_invalid_version failures #68420
• cmd/go: TestScript/mod_invalid_version fails on Windows #46691 (closed)
• cmd/go: TestScript/mod_invalid_version failing as of CL 188817 #33474 (closed)
• cmd/go: TestScript/mod_invalid_version failing on linux-amd64-longtest and linux-386-longtest builders #47444 (closed)
• cmd/go: TestScript failures #64568 (closed)
• cmd/go: TestScript/mod_invalid_version is failing locally #47233 (closed)
• cmd/go: tests fail without network #35703 (closed)
• cmd/go: TestScript/mod_invalid_version failures on linux-386-longtest due to missing ancestor tags #52796 (closed)
• cmd/go: TestScript/mod_get_fallback failures due to unexpected 404s from proxy.golang.org #65459

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[matloob]

This is one of the tests that connects to the internet so it's inherently flaky. We should try to see if we can get it to use the local vcstest server instead.