x/vuln: TestGovulncheck failures due to timeout dialing `vuln.go.dev`

[gopherbot]

#!watchflakes
post <- pkg == "golang.org/x/vuln" && test == "TestGovulncheck" && `Get "https://vuln.go.dev/.*": dial tcp .*: i/o timeout`

Issue created automatically to collect these failures.

Example (log):

go mod tidy

--- FAIL: TestGovulncheck (61.38s)
    all_test.go:94: govulncheck finished with std out/err:
        Scanning your code and 207 packages across 12 dependent modules for known vulnerabilities...

    all_test.go:103: unrecognized govulncheck command error: govulncheck: Get "https://vuln.go.dev/index/modules.json.gz": dial tcp 34.117.213.18:443: i/o timeout

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "golang.org/x/vuln" && test == "TestGovulncheck"

2023-12-05 20:09 linux-arm64-longtest vuln@c27f855e go@018c1da0 x/vuln.TestGovulncheck (log)

go mod tidy

--- FAIL: TestGovulncheck (61.38s)
    all_test.go:94: govulncheck finished with std out/err:
        Scanning your code and 207 packages across 12 dependent modules for known vulnerabilities...

    all_test.go:103: unrecognized govulncheck command error: govulncheck: Get "https://vuln.go.dev/index/modules.json.gz": dial tcp 34.117.213.18:443: i/o timeout

— watchflakes

[gopherbot]

Change https://go.dev/cl/549375 mentions this issue: internal/vulncheck: improve error message for fetching vulns