build: get readline/libiconv from backup storage

Download the readline and libiconv archives from the backup storage to
avoid network issues with accessing https://ftp.gnu.org in the future.

NO_DOC=build
NO_TEST=build
NO_CHANGELOG=build

(cherry picked from commit e41c47a8)

static-build/cmake/AddDependencyProjects.cmake

@@ -187,7 +187,7 @@ set(TARANTOOL_DEPENDS ncurses ${TARANTOOL_DEPENDS})
 # Patched to fix file descriptor leak with zero-length history file.
 #
 ExternalProject_Add(readline
-    URL https://ftp.gnu.org/gnu/readline/readline-${READLINE_VERSION}.tar.gz
+    URL ${BACKUP_STORAGE}/readline/readline-${READLINE_VERSION}.tar.gz
     URL_MD5 ${READLINE_HASH}
     CONFIGURE_COMMAND <SOURCE_DIR>/configure
         CC=${CMAKE_C_COMPILER}
@@ -207,7 +207,7 @@ set(TARANTOOL_DEPENDS readline ${TARANTOOL_DEPENDS})
 #
 if (APPLE)
     ExternalProject_Add(iconv
-        URL https://ftp.gnu.org/pub/gnu/libiconv/libiconv-${LIBICONV_VERSION}.tar.gz
+        URL URL ${BACKUP_STORAGE}/libiconv/libiconv-${LIBICONV_VERSION}.tar.gz
         URL_MD5 ${LIBICONV_HASH}
         CONFIGURE_COMMAND <SOURCE_DIR>/configure
             CC=${CMAKE_C_COMPILER}