ci: coverage fails on debian:testing runners

[dgreatwood]

gcc produces this error, during the "coverage" phase:

geninfo: ERROR: (unused) 'exclude' pattern '/github/home/.cache/*' is unused.
Excluding file '/usr/include/c++/13/bits/shared_ptr.h'
	(use "geninfo --ignore-errors unused ..." to bypass this error)

While clang produces this error:

geninfo cmd: '/usr/bin/geninfo . --output-filename coverage.info --gcov-tool /github/home/.local/bin/cov.sh --exclude /usr/* --exclude /github/home/.cache/* --exclude */tests/* --exclude */subprojects/*'
Found LLVM gcov version 16.0.6, which emulates gcov version 4.2.0
geninfo: ERROR: Your toolchain version is too old and is no longer supported by lcov.  Please upgrade - or use an older lcov release.

There are a couple of warnings as well, which look unimportant as far as I can see.

Could someone (@Tachi107 ?) fix these errors? Otherwise I guess we could remove debian:testing from the linux.yaml matrix.

Thanks!