Update RUBY_BUILD_VERSION to v20260512 (#498)

Author: joshukraine

lib/utils.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-RUBY_BUILD_VERSION="${ASDF_RUBY_BUILD_VERSION:-v20260503}"
+RUBY_BUILD_VERSION="${ASDF_RUBY_BUILD_VERSION:-v20260512}"
 RUBY_BUILD_TAG="$RUBY_BUILD_VERSION"
 
 echoerr() {