Set rust nightly 2023-05-17 for iOS tests on CI

[coveralls]

Pull Request Test Coverage Report for Build 5012804562

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 15 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.06%) to 82.902%

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Files with Coverage Reduction	New Missed Lines	%
ergo-p2p/src/peer_info.rs	1	68.42%
ergo-p2p/src/peer_spec.rs	6	81.82%
ergotree-ir/src/serialization/types.rs	8	82.58%
<!--	Total:	15		-->

Totals
Change from base Build 5012473401:	-0.06%
Covered Lines:	9969
Relevant Lines:	12025

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💛 - Coveralls