Gitlab CI: Install glibc-debug valgrind test

Currently our gitlab CI is failing due to valgrind breakage. With
Arch stripping glibc, valgrind now requires debuginfod to be active.
However the gitlab CI system combined without our testsuite does not
retrieve these symbols, even when the appropriate environmental
variable is set.

Work around this by installing the glibc-debug package directly
using a slight kludge...  All blame for this approach is assigned
to foutrelis!

Signed-off-by: Allan McRae <allan@archlinux.org>

.gitlab-ci.yml

@@ -50,6 +50,7 @@ arch-valgrind:
   extends: .arch-test
   script:
     - pacman -Syu --needed --noconfirm valgrind
+    - pacman -U --noconfirm https://geo.mirror.pkgbuild.com/core-debug/os/x86_64/glibc-debug-$(pacman -S --print-format %v glibc)-x86_64.pkg.tar.zst
     - meson build
     - ninja -C build
     - PACTEST_VALGRIND=1 fakechroot meson test -C build