Correct syn pinning on cargo 1.48

Sadly the pinning introduced in 050f5a9
was brittle in the face of any further syn updates, and has already
broken.

Here we fix it by looking up the actual version of syn to pin.

Note that this dependency is somewhat nonsense as its actually only
a `criterion` dependency, pulled in even though we haven't set the
bench flag (as we aren't yet using `resolver = 2`).

Author: TheBlueMatt

ci/ci-tests.sh

@@ -30,7 +30,8 @@ PIN_RELEASE_DEPS # pin the release dependencies in our main workspace
 [ "$RUSTC_MINOR_VERSION" -lt 56 ] && cargo update -p quote --precise "1.0.30" --verbose
 
 # The syn crate depends on too-new proc-macro2 starting with v2.0.33, i.e., has MSRV of 1.56
-[ "$RUSTC_MINOR_VERSION" -lt 56 ] && cargo update -p syn:2.0.33 --precise "2.0.32" --verbose
+SYN_2_DEP=$(cat Cargo.lock | grep -o '"syn 2.*' | tr -d '",' | tr ' ' ':')
+[ "$RUSTC_MINOR_VERSION" -lt 56 ] && cargo update -p $SYN_2_DEP --precise "2.0.32" --verbose
 
 # The proc-macro2 crate switched to Rust edition 2021 starting with v1.0.66, i.e., has MSRV of 1.56
 [ "$RUSTC_MINOR_VERSION" -lt 56 ] && cargo update -p proc-macro2 --precise "1.0.65" --verbose