Make sure DEFAULT_TARGETS includes rust testing examples.

Otherwise tests fail:

```
    def test_plugin_start(node_factory):
        """Start a minimal plugin and ensure it is well-behaved
        """
        bin_path = Path.cwd() / "target" / "debug" / "examples" / "cln-plugin-startup"
>       l1 = node_factory.get_node(options={"plugin": str(bin_path), 'test-option': 31337})
...
error starting plugin '/home/runner/work/lightning/lightning/target/debug/examples/cln-plugin-startup': opening pipe: No such file or directory
```

Signed-off-by: Rusty Russell <[email protected]>

plugins/Makefile

@@ -95,7 +95,6 @@ ifneq ($(RUST),0)
 plugins/cln-grpc: target/${RUST_PROFILE}/cln-grpc
 	@cp $< $@
 
-DEFAULT_TARGETS += $(CLN_PLUGIN_EXAMPLES) plugins/cln-grpc
 PLUGINS += plugins/cln-grpc
 endif
 
@@ -194,4 +193,8 @@ ${CLN_PLUGIN_EXAMPLES}: ${CLN_PLUGIN_SRC}
 target/${RUST_PROFILE}/cln-grpc: ${CLN_PLUGIN_SRC}
 	cargo build ${CARGO_OPTS} --bin cln-grpc
 
+ifneq ($(RUST),0)
+DEFAULT_TARGETS += $(CLN_PLUGIN_EXAMPLES) plugins/cln-grpc
+endif
+
 include plugins/test/Makefile