ci: Pin specific nightly for cargo udeps job

This is a slightly speculative fix based on
https://github.community/t/189130 and
rust-lang/cargo#9695. Although it seems this may
have been fixed/worked around in cargo, the latest version of
`cargo-udeps` still depends on cargo 0.54.0 which could be affected.

.github/workflows/all_prs.yml

@@ -51,7 +51,7 @@ jobs:
       # Install Rust and required components
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: nightly-2021-07-05
           override: true
 
       - name: Run cargo-udeps