Rollback to rust 1.77.1

* There's a joseluisq/rust-linux-darwin-builder:1.77.1 image, but no
  1.77.0 image 🤷🏻‍♂️ - so need to update so we can release macos
  binaries.
* Can't upgrade to 1.78.0 because of
  tikv/pprof-rs#232

rust-toolchain.toml

@@ -13,5 +13,5 @@
 # limitations under the License.
 
 [toolchain]
-channel = "1.78.0"
+channel = "1.77.1"
 components = ["rustfmt", "clippy"]