Rewrite `build/generate-go-licenses.go` to use `go list -m -json all`
and read license files directly from the Go module cache instead of
relying on the buggy `google/go-licenses` tool.
This removes the need for CGO, GOOS=linux, and the intermediate temp
directory, while being like 100 times faster than before:
```
$ rm assets/go-licenses.json && time make assets/go-licenses.json
go run build/generate-go-licenses.go assets/go-licenses.json
make assets/go-licenses.json 0.21s user 0.22s system 173% cpu 0.247 total
---------
Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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