Interesting. From the CI run:

A rust installation could not be found, installing via the instructions at <https://www.rustup.rs> ...
info: downloading installer
info: updating existing rustup installation

/home/travis/build/pantsbuild/pants/build-support/bootstrap_native: line 49: rustup: command not found
/home/travis/build/pantsbuild/pants/build-support/bootstrap_native: line 62: cargo: command not found

not finding the install, but then updating the install...