Make `proc_macro_span` optional with nightly

From the documentation of [`proc_macro_span`](https://github.com/dtolnay/proc-macro2/blob/fecb02df0e2966c7eb39e020dbf650fa8bafd0c5/build.rs#L15C1-L20C3) I was understanding that if you don't allow unstable features in your nightly build, then the build of the crate should still work:
"Enabled when building with nightly, unless `-Z allow-feature` in RUSTFLAGS disallows unstable features."

I am using the following configuration in my `.cargo/config.toml`:
```toml
[unstable]
allow-features = []
```

And the following rust nightly: `nightly-2023-09-25`.

When building a dummy crate that just has proc-macro2 as a dependency, the following happens:
```
cargo build --verbose
Dirty proc-macro2 v1.0.67: the config settings changed
Compiling proc-macro2 v1.0.67
Dirty unicode-ident v1.0.12: the config settings changed
Compiling unicode-ident v1.0.12
Running `/Users/fandree/.rustup/toolchains/nightly-2023-09-25-aarch64-apple-darwin/bin/rustc --crate-name build_script_build --edition=2021 /Users/fandree/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.67/build.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=312 -Zallow-features= --crate-type bin --emit=dep-info,link -C embed-bitcode=no --cfg 'feature="default"' --cfg 'feature="proc-macro"' -C metadata=643d0f678a6ab910 -C extra-filename=-643d0f678a6ab910 --out-dir /Users/fandree/sources/rust-crates/test-crate/target/debug/build/proc-macro2-643d0f678a6ab910 -L dependency=/Users/fandree/sources/rust-crates/test-crate/target/debug/deps --cap-lints allow`
Running `/Users/fandree/.rustup/toolchains/nightly-2023-09-25-aarch64-apple-darwin/bin/rustc --crate-name unicode_ident --edition=2018 /Users/fandree/.cargo/registry/src/index.crates.io-6f17d22bba15001f/unicode-ident-1.0.12/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=312 -Zallow-features= --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked -C metadata=19688c2ad055990d -C extra-filename=-19688c2ad055990d --out-dir /Users/fandree/sources/rust-crates/test-crate/target/debug/deps -L dependency=/Users/fandree/sources/rust-crates/test-crate/target/debug/deps --cap-lints allow`
Running `/Users/fandree/sources/rust-crates/test-crate/target/debug/build/proc-macro2-643d0f678a6ab910/build-script-build`
Running `/Users/fandree/.rustup/toolchains/nightly-2023-09-25-aarch64-apple-darwin/bin/rustc --crate-name proc_macro2 --edition=2021 /Users/fandree/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.67/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=312 -Zallow-features= --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked --cfg 'feature="default"' --cfg 'feature="proc-macro"' -C metadata=aef881fdd2157960 -C extra-filename=-aef881fdd2157960 --out-dir /Users/fandree/sources/rust-crates/test-crate/target/debug/deps -L dependency=/Users/fandree/sources/rust-crates/test-crate/target/debug/deps --extern unicode_ident=/Users/fandree/sources/rust-crates/test-crate/target/debug/deps/libunicode_ident-19688c2ad055990d.rmeta --cap-lints allow --cfg wrap_proc_macro --cfg proc_macro_span`
error[E0725]: the feature `proc_macro_span` is not in the list of allowed features
--> /Users/fandree/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.67/src/lib.rs:90:59
|
90 | #![cfg_attr(any(proc_macro_span, super_unstable), feature(proc_macro_span))]
```

The `--cfg proc_macro_span` was added even though we also pass the `-Zallow-features=` flag. Is this the expected behavior?

I would like to be able to build proc-macro2 with nightly without having to enable the unstable feature. Is this possible?