Revert Rust 1.68.0 upgrade (#30897)

Specifically the nightly upgrade is breaking the
`geyser_plugin_manager`.  We will probably need more investigation
before we can move forward with it.

ci/docker-rust-nightly/Dockerfile

@@ -1,4 +1,4 @@
-FROM solanalabs/rust:1.68.0
+FROM solanalabs/rust:1.67.1
 ARG date
 
 RUN set -x \

ci/docker-rust/Dockerfile

@@ -1,6 +1,6 @@
 # Note: when the rust version is changed also modify
 # ci/rust-version.sh to pick up the new image tag
-FROM rust:1.68.0
+FROM rust:1.67.1
 
 RUN set -x \
  && apt update \

ci/rust-version.sh

@@ -29,7 +29,7 @@ fi
 if [[ -n $RUST_NIGHTLY_VERSION ]]; then
   nightly_version="$RUST_NIGHTLY_VERSION"
 else
-  nightly_version=2023-02-08
+  nightly_version=2023-01-22
 fi
 
 

rust-toolchain.toml

@@ -1,2 +1,2 @@
 [toolchain]
-channel = "1.68.0"
+channel = "1.67.1"