ci: compute run code coverage only if explicitly requested by variable

ci: compute run code coverage only if explicitly requested by variable

Sometimes, for a unknown reason, coverage computation fails in Deckard pipeline:

$ MAKEFLAGS="--jobs $(nproc)" make coverage-c coverage-lua COVERAGE_STAGE=gcov-deckard 2>&1 | grep -vE '(source file is newer than notes file)|(the message is displayed only once per source file)'
lcov: Need one of options -z, -c, -a, -e, -r, -l, --diff or --summary
Use lcov --help to get usage information
coverage.mk:15: recipe for target 'coverage-c' failed
make: *** [coverage-c] Error 255

This is now normally disabled not to interfere with normal development.