Use correct PATH separator for Cygwin in pg_regress.c.

This has been broken for years, and I'm not sure why it has not been
noticed before, but now a very modern Cygwin breaks on it, and the fix
is clearly correct. Backpatching to all live branches.

src/test/regress/pg_regress.c

@@ -817,8 +817,10 @@ initialize_environment(void)
 		add_to_path("LD_LIBRARY_PATH", ':', libdir);
 		add_to_path("DYLD_LIBRARY_PATH", ':', libdir);
 		add_to_path("LIBPATH", ':', libdir);
-#if defined(WIN32) || defined(__CYGWIN__)
+#if defined(WIN32)
 		add_to_path("PATH", ';', libdir);
+#elif defined(__CYGWIN__)
+		add_to_path("PATH", ':', libdir);
 #endif
 	}
 	else