update rust toolchain

Summary:
It includes this fix which was backported to 1.62.1.
rust-lang/rust#98950

`2022-08-06` is what will become 1.64.0.

Changes are caused by:
- Split TypeVisitable from TypeFoldable: rust-lang/rust#98206
- Remove `fn backtrace` and replace with usages of provider API: rust-lang/rust#99431

Reviewed By: stepancheg

Differential Revision: D39277984

fbshipit-source-id: 5afc6c2b9e5ee3074ea0ede995868aef12cebf14

buck2_common/src/result.rs

@@ -68,10 +68,6 @@ impl std::error::Error for SharedError {
         self.0.source()
     }
 
-    fn backtrace(&self) -> Option<&std::backtrace::Backtrace> {
-        Some(self.0.backtrace())
-    }
-
     fn cause(&self) -> Option<&dyn std::error::Error> {
         #[allow(deprecated)]
         self.0.cause()

rust-toolchain

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "nightly-2022-06-25"
+channel = "nightly-2022-08-06"
 components = ["llvm-tools-preview","rustc-dev"]