Merge rust-bitcoin#623: CI: Pin dependencies required for MSRV build

0e0dcb7 CI: Pin dependencies required for MSRV build (Tobin C. Harding)

Pull request description:

  Whinge, whinge, whinge, and pin the dependencies.

ACKs for top commit:
  apoelstra:
    ACK 0e0dcb7

Tree-SHA512: 3eb65a844f632ea324705fe371c3876b96d62ddabe17085ed1bf32183eb491ff5006836c3945f4b0e11886049e9bd62245d2eb2c646cbf1a2f10cbff5c54bd53

contrib/test.sh

@@ -19,6 +19,13 @@ cargo test -- --ignored --exact 'tests::test_panic_raw_ctx_should_terminate_abno
 # Make all cargo invocations verbose
 export CARGO_TERM_VERBOSE=true
 
+# Pin dependencies as required if we are using MSRV toolchain.
+if cargo --version | grep "1\.48"; then
+    # 1.0.157 uses syn 2.0 which requires edition 2021
+    cargo update -p serde_json --precise 1.0.99
+    cargo update -p serde --precise 1.0.156
+fi
+
 # Defaults / sanity checks
 cargo build --all
 cargo test --all