readme: Fix `GST_PLUGIN_PATH` instructions to include target triple.
This change also puts the `target/` directory first, for locally-built
plugins to take priority over distribution-provided plugins. It also
encloses the environment variable in quotes, in case it contains paths with
spaces.
@@ -101,9 +101,11 @@ Then you can easily build and test a specific plugin:
```
$ cargo cbuild --manifest-path video/cdg/Cargo.toml
-$ GST_PLUGIN_PATH=$GST_PLUGIN_PATH:target/debug gst-inspect-1.0 cdgdec
+$ GST_PLUGIN_PATH="target/x86_64-unknown-linux-gnu/debug:$GST_PLUGIN_PATH" gst-inspect-1.0 cdgdec
```
+Replace `x86_64-unknown-linux-gnu` with your system's Rust target triple (`rustc -vV`).
+
The plugin can also be installed system-wide:
```
