[CI] keep history of deb packages

During deployment, the deb file is saved and an history of 5 previous
deb packages are also kept.

.gitlab-ci.yml

@@ -142,7 +142,10 @@ deploy_daily:
     - cd ${INSTALL_DIR}
     - find . -maxdepth 1 ! -name "${PKG_NAME}" ! -name "." -exec rm -rf {} +
     - dpkg -x ${PKG_NAME} .
-    - rm ${PKG_NAME}
+    - DEB_PREFIX="mantid_ill_"
+    - rm ${DEB_PREFIX}5.deb
+    - for i in {4..0}; do mv ${DEB_PREFIX}$i.deb ${DEB_PREFIX}$(($i+1)).deb"; done
+    - mv ${PKG_NAME} ${DEB_PREFIX}0.deb
 
 deploy_nightly:
   stage: deployment