Using your same argument (which I do not agree with) you call cargo on the command line, so the file should be called cargo.toml.
Cargo is a proper noun, it should be capitalized, Python has Pip (setup.py), Java has Maven (pom.xml), Gradle (gradlew.bat), Ant (build.xml), etc. but as you see, they are all lowercase files.
Rust does not, i repeat not, try to be different. The whole point of Rust is that it does nothing new, but stands on the shoulders of giants, so to speak. Lifetimes are from Cyclone, much of the syntax is from ML- or C-like languages. Rust is familiar.
So you are saying we should be different for the sake of being different? All of the configuration files that I have seen with an extension start lowercase. The argument that brought up was to group it with configuration files. This is from the *file precedent, and does not belong with the style we are using.