[build] Don't use unsupported ${var,,} syntax on some shells (fix #5082)

build.sh

@@ -245,7 +245,8 @@ else
     # New build
     if [[ "$build_n" == "n" || "$build_n" == "N" ]] ; then
         read -p "Select build type [RELEASE/debug]? "
-        if [[ "${REPLY,,}" == "debug" ]] ; then
+        REPLY=$(echo $REPLY | tr '[:upper:]' '[:lower:]')
+        if [[ "${REPLY}" == "debug" ]] ; then
             build_type=Debug
             new_build_name=aseprite-debug
         else