Regression for "`<1.88.0`-versioned nightlies" using `Span::join()`

Since https://github.com/dtolnay/proc-macro2/pull/518 (I think), the `cfg(proc_macro_span)` eponymous prob file includes a mention to `Span::local_file()`. Such API is only available on `rustc >= 1.88.0`.

So, for `1.X.0-nightly` "versioned nightly" (_i.e._, versioned `rustc/cargo` + `RUSTC_BOOTSTRAP=1`) when `X < 88`, compilation of the prob file fails, disabling _all of `proc_macro_span`'s unstable API altogether_.

Among other things, this makes `Span::join()` (when wrapping `::proc-macro`) fail to produce `Some`s, when it used to for `proc-macro2 = "<1.0.100"` versions of it: the regression.

At work we will probably make do with a toolchain bump anyways (and with a so-pinned `proc-macro2` dependency in the meantime), but I thought it could be worth pointing out anyways, should others stumble into this 🙂