Add a cmake option to disable build of runSofa app

[olivier-roussel]

Title is self-explanatory.
Follows discussion about redefining CI minimal scope.

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[alxbilger]

There is already APPLICATION_RUNSOFA. It defined implicitly when you do sofa_add_subdirectory(application runSofa runSofa ON)

[olivier-roussel]

Indeed... I guess I misunderstood what has been said yesterday.