fix: FrankenPHP build args

[dunglas]

What does this PR do?

Fix a bug in Caddy build arguments introduced in #1056.
This breaks FrankenPHP's CI: https://github.com/php/frankenphp/actions/runs/22896611170/job/66432268615

cc @henderkes

Checklist before merging

If your PR involves the changes mentioned below and completed the action, please tick the corresponding option.
If a modification is not involved, please skip it directly.

• If you modified *.php or *.json, run them locally to ensure your changes are valid:
  • composer cs-fix
  • composer analyse
  • composer test
  • bin/spc dev:sort-config
• If it's an extension or dependency update, please ensure the following:
  • Add your test combination to src/globals/test-extensions.php.
  • If adding new or fixing bugs, add commit message containing extension test or test extensions to trigger full test suite.

[Copilot]

Pull request overview

Fixes FrankenPHP’s xcaddy/Caddy build ldflags by correcting how CustomBinaryName is passed, addressing a regression introduced in #1056 that breaks FrankenPHP CI.

Changes:

• Separates CustomBinaryName=frankenphp into its own -X ldflag entry instead of being concatenated into the CustomVersion value.

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[Copilot]

Optional: there doesn’t appear to be an automated test asserting the exact formatting of the XCADDY_GO_BUILD_FLAGS string for FrankenPHP (e.g., that CustomBinaryName and CustomVersion are separate -X entries and properly quoted). This area regressed once (#1056); extracting the ldflags construction into a helper and adding a unit test for the resulting env string would help prevent future quoting/concatenation mistakes.

[henderkes]

Thanks, silly oversight. I thought we tested frankenphp build in CI too, but it was turned off.