[CI] Correct the name of the Archlinux container on Dockerhub

- Arch migrated their container from the base account to the archlinux account
- The container got severely stripped down: there is no awk, diff, which, grep
  => we need to preinstall these, otherwise the distro detection function fails

(cherry picked from commit 73739735eb636fc9db9dab2606588c7a930e2e8b)

.gitlab-ci.yml

@@ -42,7 +42,7 @@ Debian:
   <<: *distro_build
 
 Archlinux:
-  image: base/archlinux
+  image: archlinux/base
   <<: *default_config
   <<: *distro_build
 

ci/install_dependencies.sh

@@ -1,6 +1,4 @@
-#!/bin/sh
-
-set -e
+#!/bin/sh -e
 
 # Debian & derivatives don't provide binary packages of googletest
 # => have to build them ourselves
@@ -16,6 +14,12 @@ debian_build_gtest() {
     cd ..
 }
 
+# workaround for really bare-bones Archlinux containers:
+if [ -x "$(command -v pacman)" ]; then
+    pacman --noconfirm -Sy
+    pacman --noconfirm -S grep gawk sed
+fi
+
 distro_id=$(grep '^ID=' /etc/os-release|awk -F = '{print $2}'|sed 's/\"//g')
 
 case "$distro_id" in
@@ -31,7 +35,7 @@ case "$distro_id" in
 
     'arch')
         pacman --noconfirm -Sy
-        pacman --noconfirm -S gcc clang cmake make ccache expat zlib libssh curl gtest python dos2unix
+        pacman --noconfirm -S gcc clang cmake make ccache expat zlib libssh curl gtest python dos2unix which diffutils
         ;;
 
     'ubuntu')