gitlab-ci: use a custom fixed build directory to be able to use any runner (among shared runners)

.gitlab/common.yml

@@ -4,10 +4,11 @@
 ---
 default:
   image: $CI_REGISTRY_IMAGE
-  tags: ['ci.inria.fr', 'linux', 'small'] # gitlab instance runner (i.e. docker on linux)
+  tags: ['ci.inria.fr', 'linux'] # gitlab instance runner (i.e. docker on linux)
   interruptible: true
 
 variables:
+  GIT_CLONE_PATH: $CI_BUILDS_DIR/$CI_PROJECT_NAMESPACE/$CI_PROJECT_NAME
   GIT_SUBMODULE_STRATEGY: recursive
 
 # push event: executed only if the source branch is the default one on