Revert "Build mpdecimal from source to workaround unreliable launchpad.net (#149248)"

This reverts commit 60b751c.

https://status.canonical.com/ is green, I think it's time
to restore old workaround.

Author: skirpichev

.github/workflows/posix-deps-apt.sh

@@ -26,16 +26,9 @@ apt-get -yq --no-install-recommends install \
     xvfb \
     zlib1g-dev
 
-# Workaround missing libmpdec-dev on ubuntu 24.04 by building mpdecimal
-# from source. ppa:ondrej/php (launchpad.net) are unreliable
-# (https://status.canonical.com) so fetch the tarball directly
-# from the upstream host.
-# https://www.bytereef.org/mpdecimal/
-MPDECIMAL_VERSION=4.0.1
-curl -fsSL "https://www.bytereef.org/software/mpdecimal/releases/mpdecimal-${MPDECIMAL_VERSION}.tar.gz" \
-    | tar -xz -C /tmp
-(cd "/tmp/mpdecimal-${MPDECIMAL_VERSION}" \
-    && ./configure --prefix=/usr/local \
-    && make -j"$(nproc)" \
-    && make install)
-ldconfig
+# Workaround missing libmpdec-dev on ubuntu 24.04:
+# https://launchpad.net/~ondrej/+archive/ubuntu/php
+# https://deb.sury.org/
+sudo add-apt-repository ppa:ondrej/php
+apt-get update
+apt-get -yq --no-install-recommends install libmpdec-dev