Don't overwrite EXTRA_INSTALL

The temp-install target sets EXTRA_INSTALL to install the current
directory.  But when doing so, it should append instead of overwrite,
otherwise settings of EXTRA_INSTALL from a makefile won't take effect.
This would cause the earthdistance test to fail when called directly,
because it would miss installing the cube module.

src/makefiles/pgxs.mk

@@ -294,7 +294,7 @@ else
 check: all submake $(REGRESS_PREP)
 	$(pg_regress_check) $(REGRESS_OPTS) $(REGRESS)
 
-temp-install: EXTRA_INSTALL=$(subdir)
+temp-install: EXTRA_INSTALL+=$(subdir)
 endif
 endif # REGRESS