CI: use /etc/localtime instead of /etc/timezone

/etc/timezone is a legacy debian specific file. Let's use localtime. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1038849

• Related: #40414

.gitlab-ci.yml

@@ -14,7 +14,7 @@ variables:
 .apt-template: &apt-template
 - export LC_ALL=C.UTF-8
 - export DEBIAN_FRONTEND=noninteractive
-- echo Etc/UTC > /etc/timezone
+- ln -fs /usr/share/zoneinfo/Etc/UTC /etc/localtime
 - echo 'quiet "1";'
        'APT::Install-Recommends "0";'
        'APT::Install-Suggests "0";'