x/build: linux-amd64-wsl builder runs into "unexpected EOF" errors downloading dependencies

[dmitshur]

From https://build.golang.org/log/1d606b9af9022065aa9b7b4dd76799415ae5fd0b:

[...]
go: downloading github.com/gobwas/pool v0.2.1
go: downloading golang.org/x/text v0.3.6
go: downloading github.com/golang/groupcache v0.0.0-20200121045136-8c9f03a8e57e
go: downloading golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1
../../../../pkg/mod/cloud.google.com/go/datastore@v1.2.0/client.go:25:2: google.golang.org/genproto@v0.0.0-20210726143408-b02e89920bf0: read "https:/goproxy.io/@v/v0.0.0-20210726143408-b02e89920bf0.zip": unexpected EOF
../../../../pkg/mod/cloud.google.com/go@v0.88.0/internal/trace/trace.go:23:2: google.golang.org/genproto@v0.0.0-20210726143408-b02e89920bf0: read "https:/goproxy.io/@v/v0.0.0-20210726143408-b02e89920bf0.zip": unexpected EOF
../../../../pkg/mod/google.golang.org/grpc@v1.39.0/status/status.go:34:2: google.golang.org/genproto@v0.0.0-20210726143408-b02e89920bf0: read "https:/goproxy.io/@v/v0.0.0-20210726143408-b02e89920bf0.zip": unexpected EOF
../../../../pkg/mod/cloud.google.com/go/datastore@v1.2.0/save.go:26:2: google.golang.org/genproto@v0.0.0-20210726143408-b02e89920bf0: read "https:/goproxy.io/@v/v0.0.0-20210726143408-b02e89920bf0.zip": unexpected EOF
../../../../pkg/mod/cloud.google.com/go@v0.88.0/cloudbuild/apiv1/v2/cloud_build_client.go:33:2: google.golang.org/genproto@v0.0.0-20210726143408-b02e89920bf0: read "https:/goproxy.io/@v/v0.0.0-20210726143408-b02e89920bf0.zip": unexpected EOF
../../../../pkg/mod/cloud.google.com/go@v0.88.0/longrunning/autogen/operations_client.go:31:2: google.golang.org/genproto@v0.0.0-20210726143408-b02e89920bf0: read "https:/goproxy.io/@v/v0.0.0-20210726143408-b02e89920bf0.zip": unexpected EOF
../../../../pkg/mod/cloud.google.com/go@v0.88.0/iam/iam.go:30:2: google.golang.org/genproto@v0.0.0-20210726143408-b02e89920bf0: read "https:/goproxy.io/@v/v0.0.0-20210726143408-b02e89920bf0.zip": unexpected EOF
../../../../pkg/mod/cloud.google.com/go/storage@v1.10.0/iam.go:24:2: google.golang.org/genproto@v0.0.0-20210726143408-b02e89920bf0: read "https:/goproxy.io/@v/v0.0.0-20210726143408-b02e89920bf0.zip": unexpected EOF

The "https:/goproxy.io" part looks unusual, should that be "https://" instead of "https:/"? CC @mengzhuo.

Possibly related to #52793, #52727, #42971, but they're all closed.

[mengzhuo]

That's weird, the logs shows that enviroment is GOPROXY=https://goproxy.io,direct

[mengzhuo]

cc @oiooj