rust: Update nightly version

This ensures that after the 2020.04 release the supported stable version
can be bumped to Rust 1.61. Using Rust 1.61 to its full potential will
require removing feature declarations, and if riot-wrappers has those
features removed, building it needs a recent nightly (like this) that
does not need them declared.

riotbuild/Dockerfile

@@ -292,7 +292,7 @@ ENV RUSTUP_HOME /opt/rustup/.rustup
 RUN \
     RUSTUP_HOME=/opt/rustup/.rustup \
     CARGO_HOME=/opt/rustup/.cargo sh -c "\
-    curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal --default-toolchain nightly-2022-01-07 && \
+    curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal --default-toolchain nightly-2022-03-08 && \
     rustup component add rust-src && \
     rustup target add i686-unknown-linux-gnu && \
     rustup target add riscv32imac-unknown-none-elf && \