ci: Pin no-std test to nightly-2023-08-05

rust-lang/rust#98333 broke RISC-V targets
without A-extension.
This will be fixed by rust-lang/rust#114497 or
rust-lang/rust#114499.

```
  = note: rust-lld: error: undefined symbol: __atomic_load_4
          >>> referenced by uint_macros.rs:1230 (/home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/uint_macros.rs:1230)
          >>>               compiler_builtins-a15f77f0f647aa99.compiler_builtins.eedcbccd0d1b9b88-cgu.1.rcgu.o:(compiler_builtins::mem::memcpy::hedd00e0c59d2a943) in archive /home/runner/work/portable-atomic/portable-atomic/target/riscv32im-unknown-none-elf/debug/deps/libcompiler_builtins-a15f77f0f647aa99.rlib
```

.github/workflows/ci.yml

@@ -376,7 +376,8 @@ jobs:
         rust:
           - '1.64'
           - stable
-          - nightly
+          # TODO: https://github.com/rust-lang/rust/pull/98333 broke RISC-V targets without A-extension.
+          - nightly-2023-08-05
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps: