Networks and Sites: Don't unnecessarily switch to the current blog in `get_blog_details()`.

Props arpitgshah, djennez, acsnaterse, joostdevalk, SergeyBiryukov.
Fixes #50391.
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git-svn-id: https://core.svn.wordpress.org/trunk@47811 1a063a9b-81f0-0310-95a4-ce76da25c4cd