Revert "ci: Switch to using Gcovr instead of LCOV"

This reverts commit 2edebe3.

It turned out that neither Gcovr supports LCOV output format that
Coveralls GH action requires, nor Coveralls GH action supports JSON
format produced by gcovr --coveralls.

Signed-off-by: Daiki Ueno <ueno@gnu.org>

ci/Dockerfile

@@ -5,5 +5,5 @@ MAINTAINER Daiki Ueno <ueno@gnu.org>
 RUN dnf -y update
 RUN dnf -y install 'dnf-command(builddep)'
 RUN dnf -y builddep 'p11-kit'
-RUN dnf install -y gettext-devel git libtool make opensc openssl valgrind meson ninja-build bash-completion gcovr python-pip libasan libubsan clang-analyzer mingw64-gcc mingw64-libffi mingw64-libtasn1 wine cppcheck
+RUN dnf install -y gettext-devel git libtool make opensc openssl valgrind meson ninja-build bash-completion lcov python-pip libasan libubsan clang-analyzer mingw64-gcc mingw64-libffi mingw64-libtasn1 wine cppcheck
 RUN dnf clean all