Delete the temporary file used in buffered GiST build, after the build.

There were two bugs here: We forgot to call gistFreeBuildBuffers() function
at the end of build, and we passed interXact == true to BufFileCreateTemp,
so the file wasn't automatically cleaned up at end-of-transaction either.

src/backend/access/gist/gistbuild.c

@@ -214,6 +214,7 @@ gistbuild(PG_FUNCTION_ARGS)
 	{
 		elog(DEBUG1, "all tuples processed, emptying buffers");
 		gistEmptyAllBuffers(&buildstate);
+		gistFreeBuildBuffers(buildstate.gfbb);
 	}
 
 	/* okay, all heap tuples are indexed */

src/backend/access/gist/gistbuildbuffers.c

@@ -58,7 +58,7 @@ gistInitBuildBuffers(int pagesPerBuffer, int levelStep, int maxLevel)
 	 * Create a temporary file to hold buffer pages that are swapped out of
 	 * memory.
 	 */
-	gfbb->pfile = BufFileCreateTemp(true);
+	gfbb->pfile = BufFileCreateTemp(false);
 	gfbb->nFileBlocks = 0;
 
 	/* Initialize free page management. */