Ruby: Pin Rust to 1.54

Add a rust-toolchain file to the Ruby directory, which instructs Rustup
to install a specific version of Rust (1.54). This will be used in CI,
so any use of language features or dependencies that don't support 1.54
will result in a CI failure.

This should ensure we have a documented minimum supported rust version
and an easy to way to update it in the future (update the rust-toolchain
file).

Author: hmac

ruby/rust-toolchain

@@ -0,0 +1,6 @@
+# This file specifies the Rust version used to develop and test the Ruby
+# extractor. It is set to the lowest version of Rust we want to support.
+
+[toolchain]
+channel = "1.54"
+profile = "default"