ci: pin some more dependencies for MSRV

Author: apoelstra

contrib/test.sh

@@ -9,6 +9,9 @@ if cargo --version | grep "1\.56"; then
     cargo update -p which --precise 4.4.0
     cargo update -p byteorder --precise 1.4.3
     cargo update -p cc --precise 1.0.94
+    cargo update -p serde_json --precise 1.0.98
+    cargo update -p serde --precise 1.0.156
+    cargo update -p ppv-lite86 --precise 0.2.8
 fi
 
 if [ "$DO_FEATURE_MATRIX" = true ]