Revert "[CI] Pin nightly rust to 2021-01-05"

This reverts commit 9e97930.
Because "nightly" works (again).

Author: stappersg

Dockerfile.ci

@@ -10,8 +10,7 @@ RUN mkdir -p /code && chown avr-rust:avr-rust /code
 USER avr-rust
 
 # Install Rustup along with nightly
-# TODO: Unpin from Jan 2021 nightly. The nightlies broke midway through the month.
-RUN wget -q https://sh.rustup.rs -O /tmp/rustup.sh && sh /tmp/rustup.sh -y --profile minimal --default-toolchain nightly-2021-01-05 -c rust-src --quiet
+RUN wget -q https://sh.rustup.rs -O /tmp/rustup.sh && sh /tmp/rustup.sh -y --profile minimal --default-toolchain nightly -c rust-src --quiet
 ENV PATH=/home/avr-rust/.cargo/bin:$PATH
 
 COPY --chown=avr-rust:avr-rust . /code/blink