Drop duplicate 'en_US.UTF-8 UTF-8' from archlinux-test

https://lists.gnucash.org/logs/2022/04/12.html#T19:11:57

util/ci/actions/archlinux-test/Dockerfile

@@ -4,7 +4,7 @@ run echo "NoExtract = !*locale*/fr*/* !usr/share/i18n/locales/fr_FR*" >> /etc/pa
 
 run pacman -Syu --quiet --noconfirm glibc gcc cmake make boost python2 pkg-config gettext gtk3 guile git ninja gtest gmock sqlite3 webkit2gtk swig gwenhywfar aqbanking intltool libxslt postgresql-libs libmariadbclient libdbi libdbi-drivers wayland-protocols > /dev/null
 
-run echo en_US.UTF-8 UTF-8 >> /etc/locale.gen
+run echo en_US.UTF-8 UTF-8 > /etc/locale.gen
 run echo en_GB.UTF-8 UTF-8 >> /etc/locale.gen
 run echo fr_FR.UTF-8 UTF-8 >> /etc/locale.gen
 run locale-gen