Fix paths for shared libraries in Dockerfile to use PG_MAJOR variable

Author: m3hm3t

circleci/images/exttester/Dockerfile

@@ -75,8 +75,8 @@ ARG PG_VERSION_CLEAN
 ARG PG_MAJOR
 
 COPY --from=dev-tools-builder \
-     /build/postgresql-${PG_VERSION_CLEAN}/build/lib/*.so \
-     /build/postgresql-${PG_VERSION_CLEAN}/build/lib/
+     /build/postgresql-${PG_MAJOR}/build/lib/*.so \
+     /build/postgresql-${PG_MAJOR}/build/lib/
 
 COPY --from=dev-tools-builder \
   /build/postgresql-${PG_VERSION_CLEAN}/build/src/test/isolation/pg_isolation_regress \