[derive] Pin syn to 2.0.55

This is a temporary work-around for #1085. Per #1088, we will remove
this and implement a more complete fix before releasing 0.8 (#671).

zerocopy-derive/Cargo.toml

@@ -29,7 +29,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0.1"
 quote = "1.0.10"
-syn = "2.0.31"
+syn = "=2.0.55"
 
 [dev-dependencies]
 # We don't use this directly, but trybuild does. On the MSRV toolchain, the