Pin serde_json in CI to fix MSRV

Unfortunately `serde_json` switched to use Rust edition 2021 with
version 1.0.101, i.e., has an MSRV of 1.56 now.

Author: tnull

ci/ci-tests.sh

@@ -11,6 +11,9 @@ HOST_PLATFORM="$(rustc --version --verbose | grep "host:" | awk '{ print $2 }')"
 # Sadly the log crate is always a dependency of tokio until 1.20, and has no reasonable MSRV guarantees
 [ "$RUSTC_MINOR_VERSION" -lt 49 ] && cargo update -p log --precise "0.4.18" --verbose
 
+# serde_json 1.0.101 uses rust edition 2021, i.e., has MSRV of 1.56
+[ "$RUSTC_MINOR_VERSION" -lt 56 ] && cargo update -p serde_json --precise "1.0.100" --verbose
+
 # The addr2line v0.20 crate (a dependency of `backtrace` starting with 0.3.68) relies on 1.55+
 [ "$RUSTC_MINOR_VERSION" -lt 55 ] && cargo update -p backtrace --precise "0.3.67" --verbose