Pin aws-lc-rs integ to nightly-2024-05-22 (#1612)

### Description of changes: 
* Pin to a working Rust nightly toolchain.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

.github/workflows/aws-lc-rs.yml

@@ -10,6 +10,7 @@ concurrency:
 env:
   GOPROXY: https://proxy.golang.org,direct
   AWS_LC_SYS_CMAKE_BUILDER: 1
+  RUST_NIGHTLY_TOOLCHAIN: nightly-2024-05-22
 jobs:
   standard:
     if: github.repository_owner == 'aws'
@@ -20,11 +21,11 @@ jobs:
           repository: awslabs/aws-lc-rs
           path: ./aws-lc-rs
           submodules: false
-      - uses: actions-rs/toolchain@v1
+      - uses: dtolnay/rust-toolchain@master
         with:
           # Our aws-lc-sys generation scripts require nightly.
-          toolchain: nightly
-          override: true
+          toolchain: ${{ env.RUST_NIGHTLY_TOOLCHAIN }}
+      - run: rustup override set $RUST_NIGHTLY_TOOLCHAIN
       - uses: actions-rs/cargo@v1
         with:
           command: install