Cargo build hangs with ” Blocking waiting for file lock on the registry index” after building parity from source

I followed the readme instructions for building Parity from source and then I execute this command:

cargo build --release

and then:

~/.cargo/bin/cargo build --release

.. as instructed; both of which cause me to receive the following message while the prompt hangs:

 Blocking waiting for file lock on the registry index

I'm on a Mac.