test: pin 'half' dependency on 1.41.1.

This is only needed for the serde test, so don't bother putting it in
the README. Downstream users won't encounter this dependency and don't
need to care about it.

Author: apoelstra

contrib/test.sh

@@ -13,6 +13,11 @@ if cargo --version | grep nightly; then
     NIGHTLY=true
 fi
 
+if cargo --version | grep "1\.41"; then
+    # 1.8.x uses constfns which aren't supported in 1.41
+    cargo update -p half --precise 1.7.0
+fi
+
 # Test if panic in C code aborts the process (either with a real panic or with SIGILL)
 cargo test -- --ignored --exact 'tests::test_panic_raw_ctx_should_terminate_abnormally' 2>&1 | tee /dev/stderr | grep "SIGILL\\|panicked at '\[libsecp256k1\]"