Bump Rust nightly to 2020-05-31

[kostko]

It seems that the previous nightly had some problems with runtime builds.

[codecov]

Codecov Report

Merging #2962 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2962      +/-   ##
==========================================
- Coverage   68.37%   68.33%   -0.04%     
==========================================
  Files         361      361              
  Lines       35220    35220              
==========================================
- Hits        24083    24069      -14     
- Misses       8035     8043       +8     
- Partials     3102     3108       +6     

Impacted Files	Coverage Δ
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/common/cbor/codec.go	78.37% <0.00%> (-5.41%)	⬇️
go/consensus/tendermint/abci/timer.go	67.00% <0.00%> (-5.00%)	⬇️
go/runtime/tagindexer/tagindexer.go	68.47% <0.00%> (-4.35%)	⬇️
go/storage/mkvs/lookup.go	73.46% <0.00%> (-4.09%)	⬇️
go/storage/mkvs/remove.go	85.85% <0.00%> (-4.05%)	⬇️
go/storage/mkvs/iterator.go	83.12% <0.00%> (-2.50%)	⬇️
go/runtime/transaction/transaction.go	74.54% <0.00%> (-2.43%)	⬇️
go/worker/compute/txnscheduler/committee/node.go	62.76% <0.00%> (-2.11%)	⬇️
... and 23 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2761bf8...2fedfbb. Read the comment docs.