utils/analytics: improve accuracy of GitHub Packages download counts

[carlocab]

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---

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.

[MikeMcQuaid]

Nice work @carlocab!