ci: switch back to meson-0.56.2

It seems there is another meson (0.57.0) regression preventing clang from
building systemd with --optimization=3 -Db_lto=true

By analogy with google/oss-fuzz#5199 let's just switch
to 0.56.2 for the time being

.github/workflows/build_test.sh

@@ -95,7 +95,7 @@ apt-get -y install "${PACKAGES[@]}"
 # support all the features we need (like --optimization=). Since the build-dep
 # command above installs the distro versions, let's install the pip ones just
 # locally and add the local bin directory to the $PATH.
-pip3 install --user -U meson ninja
+pip3 install --user -U meson==0.56.2 ninja
 export PATH="$HOME/.local/bin:$PATH"
 
 $CC --version