[CI]Remove CC=gcc for COVERAGE build in Makefile

norvellrequested to merge
ci/fix-coverage-build into main

The coverage job failed in the BASOP since the Makefile update to run clang-18. I think the problem is that the Makefile in BASOP still sets gcc for COVERAGE build.

Worked on Nokia Linux runner in PC float repo

https://forge.3gpp.org/rep/ivas-codec-pc/ivas-codec/-/jobs/746338

Failed on Ericsson Linux runner 5 in BASOP repo

https://forge.3gpp.org/rep/sa4/audio/ivas-basop/-/jobs/746323

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