create: detect URL from latest release if it isn't archive

[abitrolly]

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When running brew create https://github.com/buildpacks/pack the version is not detected from the URL, so the version from the latest release is used. But the URL still doesn't point to the source code archive. This PR constructs the URL to point to the source archive for the detected release or tag.

[MikeMcQuaid]

• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

Please always do these before opening a PR.

There's legit failures in https://github.com/Homebrew/brew/actions/runs/15821654438/job/44592172254?pr=20156

[abitrolly]

@MikeMcQuaid I simplified the logic. If version is fetched from the latest release, the URL also made to point to its source code. Without additional fetch.

[MikeMcQuaid]

@abitrolly Thanks!