GitHub Actions: Disable LTO for macOS builds

[ulmus-scott]

This was causing TestCopyFrames to abort after
passing all of the tests on macOS 15.

Checklist

• contribution does not duplicate one of our existing pull requests
• contribution is in a branch rebased against master
• code compiles successfully without errors
• code follows the MythTV Coding Standards
• documentation added/updated/removed where necessary
• commits are logically organised and have good commit messages

[jhoyt4]

thank you for fixing this. It baffled me months ago when it first appeared and I couldn't get the issue to occur on my home machines.

[ulmus-scott]

Looking at the GitHub Actions history is seems 6a1e3c3 enabled LTO for the macOS builders.

It aborts after the tests have completed successfully, but I have no idea why. (This is what the merged changes from #1101 show.)

Since LTO isn't necessary, I see no reason not to disable it if it is causing problems.

[ulmus-scott]

This disables LTO on all macOS builders, not just the failing macOS 15, since that was easier to do.

[jhoyt4]

makes sense to me. I can keep an eye on the older macOS versions and re-enable it if issues arise.