Pin the toolchain version to 2021-01-07

Cryptjar · Cryptjar · commit fa8d78fac669 · 2022-04-02T03:46:53.000+02:00
At the time of writing this is latest Rust version that works with AVR.
diff --git a/rust-toolchain.toml b/rust-toolchain.toml
@@ -0,0 +1,4 @@
+[toolchain]
+channel = "nightly-2021-01-07"
+profile = "minimal"
+components = ["rustfmt", "rust-src"]
