ci: Fix gcovr bug when assembling coverage out of gcno files

Ubuntu 20.04 ships gcovr 4.2, which seems unable to assemble coverage
files out of gcda/gcno files. For some reason it returns files with
0% lines always.

Assembling the files manually (as was done previously, running gcov
on each pair of gcda/gcno, then calling gcov) would be enough but
only if passed the -g flag. So instead let's follow macOS and Windows,
and use gcovr 6.0 through pip.

.github/workflows/workflow.yml

@@ -61,7 +61,7 @@ jobs:
           CXX=${CC/#gcc/g++}
           sudo apt-add-repository ppa:ubuntu-toolchain-r/test
           sudo apt-get update
-          sudo apt-get install $CC $CXX gcovr
+          sudo apt-get install $CC $CXX
           echo "CC=$CC" >> $GITHUB_ENV
           echo "CXX=$CXX" >> $GITHUB_ENV
           echo "GCOV=${CC/#gcc/gcov}" >> $GITHUB_ENV
@@ -75,7 +75,7 @@ jobs:
           rm llvm-snapshot.gpg.key
           sudo apt-add-repository "deb https://apt.llvm.org/${{ matrix.os.name }}/ llvm-toolchain-${{ matrix.os.name }}${CC/#clang/} main"
           sudo apt-get update
-          sudo apt-get install $CC gcovr
+          sudo apt-get install $CC
           CXX=${CC/#clang/clang++}
           echo "CC=$CC" >> $GITHUB_ENV
           echo "CXX=$CXX" >> $GITHUB_ENV
@@ -95,10 +95,10 @@ jobs:
         with:
           lfs: true
           submodules: true
-      - name: Setup Meson + Ninja
+      - name: Setup Meson + Ninja + gcovr
         run: |
           sudo python3 -m pip install --upgrade pip setuptools wheel
-          sudo python3 -m pip install meson ninja
+          sudo python3 -m pip install meson ninja gcovr
         working-directory: ${{ runner.temp }}
       - name: Version tools
         run: |