cmd/go: error from fetching go.mod checksum not diagnosed by `go list -mod=mod`

[bcmills]

#!watchflakes
post <- pkg == "cmd/go" && log ~ `FAIL: TestScript/mod_sumdb_golang` && `go.sum and saved.sum differ`

(Pulled out from #64021 (comment).)

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- pkg == "cmd/go" && log ~ `FAIL: TestScript/mod_sumdb_golang` && `go.sum and saved.sum differ`

2023-10-09 22:05 linux-amd64-longtest go@8f61fab6 cmd/go.TestScript (log)

vcs-test.golang.org rerouted to http://127.0.0.1:38465
https://vcs-test.golang.org rerouted to https://127.0.0.1:35057
go test proxy running at GOPROXY=http://127.0.0.1:46193/mod
2023/10/09 22:40:58 http: TLS handshake error from 127.0.0.1:34474: EOF
--- FAIL: TestScript (0.07s)
    --- FAIL: TestScript/mod_sumdb_golang (80.32s)
        script_test.go:132: 2023-10-09T22:40:01Z
        script_test.go:134: $WORK=/workdir/tmp/cmd-go-test-1346709016/tmpdir1346512070/mod_sumdb_golang2533510841
        script_test.go:156: 
            # Test default GOPROXY and GOSUMDB (0.219s)
...
            +++ saved.sum
            @@ -1,5 +1,6 @@
             golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:qgOY6WgZOaTkIIMiVjBQcw93ERBE4m30iBm00nkL0i8=
             golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
            +golang.org/x/text v0.13.0/go.mod h1:TvPlkZtksWOMsz7fbANvkp4WM8x/WCo/om8BMLbz+aE=
             rsc.io/quote v1.5.2 h1:w5fcysjrx7yqtD/aO+QwRjYZOKnaM9Uh2b40tElTs3Y=
             rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
             rsc.io/sampler v1.3.0 h1:7uVkIFmeBqHfdjD+gZwtXXI+RODJ2Wc4O7MPEh/QiW4=

        script_test.go:156: FAIL: testdata/script/mod_sumdb_golang.txt:62: cmp go.sum saved.sum: go.sum and saved.sum differ

— watchflakes