Bump Rust toolchain

thomaseizinger · thomaseizinger · commit 45fc0a9c80dc · 2021-03-22T13:40:49.000+11:00
This should fix rust-lang/socket2#213.
diff --git a/rust-toolchain b/rust-toolchain
@@ -1,4 +1,4 @@
 [toolchain]
-channel = "nightly-2021-01-31"
+channel = "nightly-2021-03-21"
 components = ["rustfmt", "clippy"]
 targets = ["armv7-unknown-linux-gnueabihf"]
