Cargo build system: Lock file cannot be updated

[Francisco Gómez García]

I’m relatively new to Guix – been casually trying it out from time to time this 
last 2 years, but never used it beyond installing Emacs and a few extra 
packages. I decided, for some reason, to learn to create packages by using the 
new Cargo build system for crates. I ended up going for `ansi-term` 
(https://crates.io/crates/ansi_term). After managing to get a working script 
(not hard at all, if you know how!), I attempt to build the package, and after 
a few seconds the following log appears and the process aborts:

  starting phase `build'
  error: the lock file needs to be updated but --locked was passed to prevent 
this
  phase `build' failed after 0.0 seconds

I can assume it’s attempting to modify the current crate's Cargo.lock, but 
obviously is unable to do so. Is this some sort of bug on the build system, or 
is there any parameter which the script is missing?

My .scm has been attached to the mail, for those who want to take a look. Thank 
you in advance!

rust-ansi-term.scm
Description: Binary data