Switch debug build to RelWithDebInfo (rapidsai#6033)

It seems cuML doesn't actually build with the `Debug` build type using the `-g` option. Switching to `RelWithDebInfo` seems to work, and gives us line number information, although some code is optimized out. Seems like a reasonable comprise to help with debugging until we can get the `Debug` working properly.

Authors:
  - Rong Ou (https://github.com/rongou)

Approvers:
  - Dante Gama Dessavre (https://github.com/dantegd)
  - Kyle Edwards (https://github.com/KyleFromNVIDIA)

URL: rapidsai#6033

build.sh

@@ -168,7 +168,7 @@ while true; do
             CMAKE_LOG_LEVEL=VERBOSE
             ;;
         -g | --debug )
-            BUILD_TYPE=Debug
+            BUILD_TYPE=RelWithDebInfo
             ;;
         -n | --no-install )
             INSTALL_TARGET=""