Pin proc-macro2 in CI to fix MSRV breakage

The proc-macro2 crate switched to Rust edition 2021 starting with
v1.0.66, i.e., has MSRV of 1.56.

Here, we pin it in CI to fix the breakage.

Author: tnull

ci/ci-tests.sh

@@ -17,6 +17,9 @@ HOST_PLATFORM="$(rustc --version --verbose | grep "host:" | awk '{ print $2 }')"
 # The serde_json crate switched to Rust edition 2021 starting with v1.0.101, i.e., has MSRV of 1.56
 [ "$RUSTC_MINOR_VERSION" -lt 56 ] && cargo update -p serde_json --precise "1.0.100" --verbose
 
+# The proc-macro2 crate switched to Rust edition 2021 starting with v1.0.66, i.e., has MSRV of 1.56
+[ "$RUSTC_MINOR_VERSION" -lt 56 ] && cargo update -p serde_json --precise "1.0.65" --verbose
+
 [ "$LDK_COVERAGE_BUILD" != "" ] && export RUSTFLAGS="-C link-dead-code"
 
 export RUST_BACKTRACE=1