New issue by adrian-bl on void-packages repository

https://github.com/void-linux/void-packages/issues/45775

Description:
### Is this a new report?

Yes

### System Info

Void 6.3.13_1 x86_64 GenuineIntel uptodate rrFFFFF

### Package(s) Affected

go-1.21.0_1

### Does a report exist for this bug with the project's home (upstream) and/or another distro?

Since go-1.21.0_1, downloading modules via `go mod tidy` fails with:

```
        github.com/google/go-cmp/cmp: GOPROXY list is not the empty string, but contains no entries
```

This seems to be due to both `GOPROXY` and `GOSUMDB` being set to an empty value (due to `/usr/lib/go/go.env` not existing?)

```
$ go env|grep -E 'PROXY|SUM'
GONOPROXY=''
GONOSUMDB=''
GOPROXY=''
GOSUMDB=''
```


Upstream-go contains the following go.env file:
https://github.com/golang/go/blob/master/go.env

and setting the same values (either by `export ..` or `go env -w ...`) fixes the issue.

### Expected behaviour

go get and go mod tidy being able to download modules

### Actual behaviour

Fails due to empty GOPROXY setting

### Steps to reproduce

* Upgrade to go1.21
* cd $project
* rm go.*
* go mod init
* go mod tidy # <- fails