Very slow when go.mod contains a new go version (go1.23) than in athens image (go1.22) - not caching

**Describe the bug**
A clear and concise description of what the bug is.

Inside the athens container, the following command takes 40s when go.mod for github.com/org/repo uses go1.23:
```
wget http://localhost:3000/github.com/org/repo/@v/hash.info -O -
```

Downgrading go.mod to to go1.22.8 and updating the hash to that commit results in a 3 second response time.

There are probably three issues here:
* Quick fix: Upgrade athens to go1.23
* Probably (not confirmed): SDK is downloaded but not cached in /var/lib/athens (when using disk cache)
* When a new Go toolchain is used, /var/lib/athens is not updated (when using disk cache)

**To Reproduce**

```go.mod
module github.com/org/repo

go 1.23.2
```

**Expected behavior**

* Fast download second time a new SDK is used (Newer SDKs cached, and module cached)

**Environment (please complete the following information):**
- OS: [e.g. Linux 64bit]
- Go version : 1.22.8/1.23
- Proxy version : 0.5.4
- Storage (fs/mongodb/s3 etc.) : fs