bootstrap: Allow --blessing changes to editor settings files

[jyn514]

Previously, on any change you would have to first edit all 4 settings files by hand, then run the tests 4 times in a row to discover what the new hashes are. After this change, you still need to edit the files by hand, but you can now run x test --bless -- hash to update the hashes without manually editing them.

[rustbot]

r? @jieyouxu

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[jieyouxu]

Thanks. One doc request, but otherwise this is a nice QoL improvement when editing setup editor configs
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[jieyouxu]

Remark: hm, I think this is only currently used for cargo insta path resolution snapshot tests, but recycling it for blessing editor hashes seems... fine.

[jieyouxu]

Suggestion: could we maybe document INSTA_UPDATE=always ==> rebless editor hashes in bootstrap setup in rustc-dev-guide somewhere? Otherwise it's really hard to discover (since INSTA_UPDATE=1 won't work).