ci: use bundled composer in CI

.gitlab-ci.yml

-image: composer
+image: php
 
 cache:
     paths:
@@ -7,9 +7,9 @@ cache:
 build:
     stage: build
     script:
-        - composer install
+        - "$CI_PROJECT_DIR/tools/composer install"
 
 test:
     stage: test
     script:
-        - composer test
+        - "$CI_PROJECT_DIR/tools/composer test"

.phive/phars.xml

@@ -5,4 +5,5 @@
   <phar name="phpcs" version="^3.6.2" installed="3.6.2" location="./tools/phpcs" copy="true"/>
   <phar name="phpcbf" version="^3.6.2" installed="3.6.2" location="./tools/phpcbf" copy="true"/>
   <phar name="grumphp" version="^1.12.0" installed="1.12.0" location="./tools/grumphp" copy="true"/>
+  <phar name="composer" version="^2.3.4" installed="2.3.4" location="./tools/composer" copy="true"/>
 </phive>