fix format warning (-Wformat-security)

[kpasko]

......./geode/geode/../geode/utility/format.h:76:24: warning: format
string is not a string literal (potentially insecure) [-Wformat-security]
return format_helper(format,format_sanitize(args)...);
^~~~~~
......./geode/geode/../geode/value/Prop.h:281:24: note: in
instantiation of function template specialization 'geode::format<>' requested here
throw ValueError(format("non-clampable prop does not have a step"));

This comes up often and is only a warning, but annoying when looking for actual compiler errors.

Either modify format_helper/format, or add -Wnoformat-security to compiler flags.