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 mod.rs:1242 (/rustc/eb088b8b9d98f1af1b0e61bbdcd8686e1b0db7b6/library/core/src/num/mod.rs:1242)
          >>>               compiler_builtins-d066fd6ed508b6b5.compiler_builtins.b1b28d926042a9f7-cgu.004.rcgu.o:(compiler_builtins::mem::memcpy::he6d5500b219c1d3d) in archive /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/riscv32i-unknown-none-elf/lib/libcompiler_builtins-d066fd6ed508b6b5.rlib
```

Author: taiki-e

.github/workflows/ci.yml

@@ -312,7 +312,8 @@ 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: