fix(opa): Use $PRODUCT_VERSION instead of $PRODUCT (#1531)

This previsouly only worked by accident, because $PRODUCT expanded
to nothing/an empty string and resulted in the complete 'patches'
folder to be copied into the image. patchale was then able to
properly patch OPA. #1524 added check=error=true to the Dockerfile
which likely causes the current failures.

This commit fixes the error by using the proper variable.

Author: Techassi

opa/Dockerfile

@@ -66,7 +66,7 @@ COPY --from=golang-image /usr/local/go/ /usr/local/go/
 ENV PATH="/usr/local/go/bin:${PATH}"
 
 COPY --chown=${STACKABLE_USER_UID}:0 opa/stackable/patches/patchable.toml /stackable/src/opa/stackable/patches/patchable.toml
-COPY --chown=${STACKABLE_USER_UID}:0 opa/stackable/patches/${PRODUCT} /stackable/src/opa/stackable/patches/${PRODUCT}
+COPY --chown=${STACKABLE_USER_UID}:0 opa/stackable/patches/${PRODUCT_VERSION} /stackable/src/opa/stackable/patches/${PRODUCT_VERSION}
 
 WORKDIR /stackable