Add rust toolchain file to specify the rust version used on CI. (#3598)

Author: Wumpf

rust-toolchain

@@ -0,0 +1,10 @@
+# If you see this, run "rustup self update" to get rustup 1.23 or newer.
+
+# NOTE: above comment is for older `rustup` (before TOML support was added),
+# which will treat the first line as the toolchain name, and therefore show it
+# to the user in the error, instead of "error: invalid channel name '[toolchain]'".
+
+[toolchain]
+channel = "1.64"
+components = ["rustfmt", "clippy"]
+targets = ["wasm32-unknown-unknown"]