CI: Pin dependencies for MSRV build ... properly

Commit `0e0dcb7f CI: Pin dependencies required for MSRV build` is
totally wrong, why did it get through CI?

In the CI script do:

- `serde_json` is not a dependency of `secp256k1`, remove the pinning
- Put the pinning _before_ any call to `cargo`
- Pin the transient dependency `wasm-bindgen-test`

contrib/test.sh

@@ -13,19 +13,18 @@ if cargo --version | grep nightly; then
     NIGHTLY=true
 fi
 
+# Pin dependencies as required if we are using MSRV toolchain.
+if cargo --version | grep "1\.48"; then
+    cargo update -p wasm-bindgen-test --precise 0.3.34
+    cargo update -p serde --precise 1.0.156
+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 '\[libsecp256k1\]"
 
 # 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