[CI] Incremental builds inadvertently disabled on all MacOS

[pcanal]

In #20072, we attempted to "[not] use incremental builds for Mac15 and Mac26", however we used:

          INCREMENTAL: ${{ !contains(github.event.pull_request.labels.*.name, 'clean build') && !matrix.platform == 'mac15' && !matrix.platform == 'mac26'}}

where the syntax is incorrectly used (the later tests parses as (!matrix.platform) == 'mac15' rather than !(matrix.platform == 'mac15'). as intended.

The consequence is that the incremental build are disable on all macOS pull_request builds rather than "just" Mac15 and Mac26.

We ought to clarify this by either only disabling on the intended plaform

          INCREMENTAL: ${{ !contains(github.event.pull_request.labels.*.name, 'clean build') && matrix.platform != 'mac15' && matrix.platform != 'mac26'}}

or hard code the disabling :):

          INCREMENTAL: false

AI disclosure: the issue was spotted by MAI-Code-1.1-Flash and verified directly (eg. see the value of INCREMENTAL in the MacOS build at https://github.com/root-project/root/actions/runs/32272212973/job/96131126275?pr=23112)