Regression in serde 1.0.157 when a proc macro enables syn's full feature

[nox]

With https://github.com/nox/serde-none-group, the command cargo build --features syn-full fails while the command cargo build succeeds. The only difference is that the failing command enables the "full" feature of the crate syn from a proc macro.

The build failure is:

± cargo build --features syn-full
   Compiling syn v2.0.2
   Compiling serde_derive v1.0.157
   Compiling another-proc-macro v0.1.67 (/Users/nox/src/serde-none-group/another-proc-macro)
   Compiling serde v1.0.157
   Compiling serde-none-group v0.1.0 (/Users/nox/src/serde-none-group)
error: expected serde crate attribute to be a string: `crate = "..."`
  --> src/lib.rs:4:25
   |
4  |           #[serde(crate = $serde_path)]
   |                           ^^^^^^^^^^^
...
9  | / bug_if_async_trait! {
10 | |     "serde"
11 | | }
   | |_- in this macro invocation
   |
   = note: this error originates in the macro `bug_if_async_trait` (in Nightly builds, run with -Z macro-backtrace for more info)

I've debugged it and the reason is that serde_derive sees a None-delimited group instead of a bare literal, but only when the "full" feature of syn is enabled.

[dtolnay]

Sorry about the breakage, this is fixed in serde_derive 1.0.158.