Fix CI

[psychon]

Fix new unused result warning

The nightly build started failing with the following message:

error: unused result of type `((), ())`
  --> xtrace-example/src/main.rs:91:5
   |
91 |     futures_util::try_join!(future1, future2)?;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

and

Pin last working unicode-ident version in msrv-check

See https://github.com/dtolnay/unicode-ident/commit/93f6598c484e61d6c4101179a9a9b52699a7e54f

and

Pin last working syn version in msrv-check

See https://github.com/dtolnay/syn/commit/92a0aa4096e5938025eb643312f2394547cae3f4

A first version if this failed with the following, so the exact version to
update is specified:

error: There are multiple `syn` packages in your project, and the specification `syn` is ambiguous.
Please re-run this command with `-p <spec>` where `<spec>` is one of the following:
  syn@1.0.109
  syn@2.0.117