utils/analytics: improve accuracy of GitHub Packages download counts

`millions_match.captures.first` will typically be a decimal (since it's
matched using `\d+\.\d+`), except we lose accuracy in the `#to_i`
conversion.

Before:
```
❯ brew info --analytics --github-packages-downloads sqlite
==> Analytics
==> install (30 days)
[snip]
==> GitHub Packages Downloads
1,009,898 (30 days)
```

After:

```
❯ brew info --analytics --github-packages-downloads sqlite
==> Analytics
==> install (30 days)
[snip]
==> GitHub Packages Downloads
1,199,898 (30 days)
```

In this case `1.19M` was being rounded down to `1M`.