coverage: fix lcov branch coverage generation
coverage: fix lcov branch coverage generation

lcov branch coverage are disabled by default.  So branch
coverage has to be enabled for every stage to make them work.

Fixes https://github.com/mesonbuild/meson/issues/4319

jpakkane/meson,mesonbuild/meson,pexip/meson,MathieuDuponchelle/meson,becm/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,becm/meson,becm/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,QuLogic/meson,QuLogic/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,QuLogic/meson,jpakkane/meson,becm/meson,MathieuDuponchelle/meson,QuLogic/meson,MathieuDuponchelle/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,becm/meson,jpakkane/meson,MathieuDuponchelle/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,becm/meson,pexip/meson,mesonbuild/meson,MathieuDuponchelle/meson,mesonbuild/meson,becm/meson,QuLogic/meson,pexip/meson,MathieuDuponchelle/meson,QuLogic/meson,becm/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,becm/meson,jpakkane/meson,mesonbuild/meson,becm/meson,pexip/meson,pexip/meson