ci(linux): drop lcov usage

I haven't been able to make lcov 2.0 work for us, so let's just drop it
and directly use gcov instead, since codecov supports it as well.

.github/workflows/linux.yaml

@@ -57,7 +57,7 @@ jobs:
       run: |
         if [ ${{ matrix.compiler }} = gcc ]; then compiler=g++; else compiler="clang lld ?exact-name(libclang-rt-dev)"; fi
         apt -y update
-        apt -y install $compiler meson pkg-config cmake rapidjson-dev libssl-dev netbase '?exact-name(libhowardhinnant-date-dev)' '?exact-name(libgmock-dev) (?version([1-9]\.[1-9][1-9]) | ?version([1-9]\.[2-9][0-9]))' '?exact-name(libcpp-httplib-dev)' libcurl4-openssl-dev git ca-certificates curl sqop lcov llvm-dev --no-install-recommends
+        apt -y install $compiler meson pkg-config cmake rapidjson-dev libssl-dev netbase '?exact-name(libhowardhinnant-date-dev)' '?exact-name(libgmock-dev) (?version([1-9]\.[1-9][1-9]) | ?version([1-9]\.[2-9][0-9]))' '?exact-name(libcpp-httplib-dev)' libcurl4-openssl-dev git ca-certificates curl sqop llvm-dev --no-install-recommends
 
     - name: Install dependencies (Red Hat)
       if: contains(matrix.os, 'redhat')
@@ -95,9 +95,16 @@ jobs:
       if: ${{ !contains(matrix.os, 'redhat') }}
       run: |
         mkdir -p $HOME/.local/bin
-        if [ "${{ matrix.compiler }}" = 'clang' ]; then printf 'llvm-cov gcov "$@"' > $HOME/.local/bin/cov.sh; else printf 'gcov "$@"' > $HOME/.local/bin/cov.sh; fi && chmod +x $HOME/.local/bin/cov.sh
-        lcov --capture --output-file coverage.info --directory . --gcov-tool $HOME/.local/bin/cov.sh --exclude '/usr/*' --exclude "${HOME}"'/.cache/*' --exclude '*/tests/*' --exclude '*/subprojects/*'
-        lcov --list coverage.info
+        echo '#!/bin/sh' > $HOME/.local/bin/cov.sh
+        if [ "${{ matrix.compiler }}" = 'clang' ]
+        then
+          echo 'llvm-cov gcov "$@"' >> $HOME/.local/bin/cov.sh
+        else
+          echo 'gcov "$@"' >> $HOME/.local/bin/cov.sh
+        fi
+        chmod +x $HOME/.local/bin/cov.sh
+        cd build
+        find .. -name '*.gcda' -o -name '*.gcno' | grep -v tests | xargs $HOME/.local/bin/cov.sh --branch-probabilities --preserve-paths --relative-only
         curl --remote-name-all \
           https://keybase.io/codecovsecurity/pgp_keys.asc \
           https://uploader.codecov.io/latest/linux/codecov \