intersphinx: cache is not updated when no files are changed on incremental build

[tk0miya]

Problem

• intersphinx: cache is not updated when no files are updated on incremental build
• see Builder.build: save env also with only new documents #5072 (it was merged once, but reverted at [1.8] Revert "Builder.build: save env also with only new documents" #5525).

Procedure to reproduce the problem

$ make html
(wait for invalidation of cache)
$ make html  # => fetching remote inventories, but not cached.

Expected results

• Remote inventories are cached even if no documents are changed.

Environment info

• OS: Mac
• Python version: 3.7.1
• Sphinx version: 1.8.1