[guix] delete rustup-init script after use

Summary: It is not needed after it has been used, and we don't want it in the source archive.

Test Plan:
contrib/guix/guix-build

tar tzvf output/x86_64-linux-gnu/bitcoin-abc-0.28.10.tar.gz | grep rust

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15635