Add SQLC_VERSION to docs

Author: kyleconroy

docs/guides/plugins.md

@@ -94,4 +94,7 @@ plugins:
   wasm:
     url: https://github.com/sqlc-dev/sqlc-gen-test/releases/download/v0.1.0/sqlc-gen-test.wasm
     sha256: 138220eae508d4b65a5a8cea555edd155eb2290daf576b7a8b96949acfeb3790
-```
+```
+
+The `SQLC_VERSION` environment variable will always be included in the
+environment, set to the version of the `sqlc` executable.