Significantly improve macos build times (i.e., github actions)

[mathgeekcoder]

• I noticed that a few github actions were taking >1 hour for macos builds.
• Most of this time was spent linking libhighs.
• This change enables UNITY_BUILD on APPLE and build time reduces to ~1 minute.

I haven't seen any issues with using it, but I noticed that it had been previously disabled. I'm not an Apple user, so please reject this PR if it doesn't make sense.

Note: This is PR is set to merge to my third-party-dependencies branch, i.e., PR #3031. I can rewrite it to work directly with latest if preferred.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.94%. Comparing base (5b8446c) to head (4d6fc81).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           third-party-dependencies    #3033   +/-   ##
=========================================================
  Coverage                     72.94%   72.94%           
=========================================================
  Files                           425      425           
  Lines                        102181   102181           
  Branches                      16443    16443           
=========================================================
+ Hits                          74535    74537    +2     
+ Misses                        27370    27368    -2     
  Partials                        276      276           

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[mathgeekcoder]

In particular look at:

build-macos: 1+ hour wall clock, 6+ hours compute effort vs 10 min wall clock, 16 minute compute effort

Helps other workflows too, but harder to see due to other bottlenecks.

[mathgeekcoder]

FYI: I also tried several alternative 'fixes':

• Reducing exported symbols
• Disabling IPO
• Static lib vs dynamic
• Using the legacy linker
• Using no-deduplication

None of them helped.

[galabovaa]

Great!

[galabovaa]

Thank you, @mathgeekcoder! Merging this. No need for a separate PR here!