ci: fix arg name

Author: axelrindle

Dockerfile

@@ -4,12 +4,12 @@ FROM php:${PHP_VERSION}-cli AS build
 
 ADD --chmod=0755 https://github.com/mlocati/docker-php-extension-installer/releases/latest/download/install-php-extensions /usr/local/bin/
 
-ARG EXTENSION=pgsql
+ARG PHP_EXTENSION=pgsql
 
 RUN mkdir /data
-RUN install-php-extensions "${EXTENSION}"
+RUN install-php-extensions "${PHP_EXTENSION}"
 RUN export EXTENSION_DIR=$(php -r 'echo ini_get("extension_dir");') && \
-    cp -v "${EXTENSION_DIR}/${EXTENSION}.so" /data
+    cp -v "${EXTENSION_DIR}/${PHP_EXTENSION}.so" /data
 
 
 FROM scratch