Fix new GIST build code to work under CLOBBER_CACHE_ALWAYS.

Can't say if this fixes *all* cases, but at least we get through
the "point" regression test now, which hyrax's last run did not.

Report: https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=hyrax&dt=2020-09-19%2021%3A27%3A23

src/backend/access/gist/gistbuild.c

@@ -408,6 +408,7 @@ gist_indexsortbuild(GISTBuildState *state)
 	 * replaced with the real root page at the end.
 	 */
 	page = palloc0(BLCKSZ);
+	RelationOpenSmgr(state->indexrel);
 	smgrextend(state->indexrel->rd_smgr, MAIN_FORKNUM, GIST_ROOT_BLKNO,
 			   page, true);
 	state->pages_allocated++;