Bump Rust toolchain to 2021-05-20

[kostko]

As advised in rust-lang/rust#85502 this should fix the problems with incremental builds.

[codecov]

Codecov Report

Merging #3955 (f66da20) into master (0e6956e) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3955      +/-   ##
==========================================
+ Coverage   67.07%   67.17%   +0.09%     
==========================================
  Files         408      408              
  Lines       41479    41479              
==========================================
+ Hits        27822    27863      +41     
+ Misses       9701     9674      -27     
+ Partials     3956     3942      -14     

Impacted Files	Coverage Δ
go/oasis-node/cmd/common/metrics/disk.go	65.38% <0.00%> (-19.24%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	78.94% <0.00%> (-10.53%)	⬇️
...nsensus/tendermint/apps/beacon/state/state_pvss.go	69.23% <0.00%> (-7.70%)	⬇️
go/consensus/tendermint/apps/beacon/genesis.go	62.50% <0.00%> (-6.25%)	⬇️
...consensus/tendermint/apps/roothash/transactions.go	66.19% <0.00%> (-3.34%)	⬇️
go/runtime/history/history.go	72.04% <0.00%> (-2.16%)	⬇️
go/worker/keymanager/worker.go	63.30% <0.00%> (-1.41%)	⬇️
go/runtime/client/submitter.go	60.40% <0.00%> (-1.35%)	⬇️
go/worker/storage/committee/checkpoint_sync.go	77.12% <0.00%> (-1.11%)	⬇️
go/worker/compute/executor/committee/node.go	69.33% <0.00%> (-0.34%)	⬇️
... and 24 more

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