Fix coverage CI job

[sfod]

Issue #, if available:

Currently, codecov CI job silently fails with the following errors (see codecov-linux CI job in the first commit in this PR):

.Segmentation fault
Problem running coverage on file: /home/runner/work/aws-c-mqtt/aws-c-mqtt/build/aws-c-mqtt/tests/CMakeFiles/aws-c-mqtt-tests.dir/v5/mqtt5_client_tests.c.gcda
Command produced error: /home/runner/work/aws-c-mqtt/aws-c-mqtt/build/aws-c-mqtt/tests/CMakeFiles/aws-c-mqtt-tests.dir/v5/mqtt5_client_tests.c.gcno:version 'A95*', prefer 'B33*'
Segmentation fault

These errors are caused by incompatibility between gcov and gcc of a specified version.

Description of changes:

Use default gcc in codecov CI job.

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