fix the rust toolchain version (#31)

this is to make sure the local version of Rust and the remote one in the
CI are in sync and avoid CIs such as the one of
#30 to fail where the
local one is working correctly.

rust-toolchain.toml

@@ -0,0 +1,3 @@
+[toolchain]
+profile = "default"
+channel = "1.76"