Set default of ROBOTOLOGY_PROJECT_TAGS to Custom

Otherwise ROBOTOLOGY_PROJECT_TAGS_CUSTOM_FILE is ignored

CMakeLists.txt

@@ -85,7 +85,7 @@ if(NOT CMAKE_CONFIGURATION_TYPES)
     set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS ${ROBOTOLOGY_BUILD_TYPES})
 endif()
 
-set(ROBOTOLOGY_PROJECT_TAGS "Stable" CACHE STRING "The tags to be used for the robotology projects: Stable, Unstable or Custom. This can be changed only before the first configuration.")
+set(ROBOTOLOGY_PROJECT_TAGS "Custom" CACHE STRING "The tags to be used for the robotology projects: Stable, Unstable or Custom. This can be changed only before the first configuration.")
 mark_as_advanced(ROBOTOLOGY_PROJECT_TAGS)
 set(ROBOTOLOGY_PROJECT_TAGS_CUSTOM_FILE "${CMAKE_CURRENT_SOURCE_DIR}/releases/2020.02.yaml" CACHE FILEPATH "If ROBOTOLOGY_PROJECT_TAGS is custom, this file will be loaded to specify the tags of the projects to use.")
 mark_as_advanced(ROBOTOLOGY_PROJECT_TAGS_CUSTOM_FILE)