Fix occasional `alloc` not found error in `format_runtime_string!` (#5632)

The macro hygiene for the `format_runtime_string!` macro was broken
since https://github.com/paritytech/polkadot-sdk/pull/5010, which
resulted in the following build error under certain circumstances:

```console
  error[E0433]: failed to resolve: use of undeclared crate or module `alloc`
      --> /home/clang/.cargo/registry/src/index.crates.io-6f17d22bba15001f/frame-benchmarking-36.0.0/src/v1.rs:1738:2
       |
  1738 | /     sp_runtime::format_runtime_string!(
  1739 | |         "\n* Pallet: {}\n\
  1740 | |         * Benchmark: {}\n\
  1741 | |         * Components: {:?}\n\
  ...    |
  1750 | |         error_message,
  1751 | |     )
       | |_____^ use of undeclared crate or module `alloc`
       |
       = note: this error originates in the macro `sp_runtime::format_runtime_string` (in Nightly builds, run with -Z macro-backtrace for more info)

  For more information about this error, try `rustc --explain E0433`.
```

This bug has been known already, but hasn't been fixed so far, see
https://github.com/paritytech/polkadot-sdk/issues/5213 and
https://substrate.stackexchange.com/questions/11786/use-of-undeclared-crate-or-module-alloc-when-upgrade-to-v1-13-0.

I have made a mini rust crate that can reproduce the bug, and it also
shows that this PR will fix the issue:
https://github.com/clangenb/sp-runtime-string-test.