rust: bump tarpaulin to 0.18.0

[ptrus]

Last part of #3629

[codecov]

Codecov Report

Merging #4197 (3f01ac2) into master (27f38aa) will decrease coverage by 0.52%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4197      +/-   ##
==========================================
- Coverage   69.26%   68.74%   -0.53%     
==========================================
  Files         411      411              
  Lines       47420    47420              
==========================================
- Hits        32847    32598     -249     
- Misses      10533    10797     +264     
+ Partials     4040     4025      -15     

Impacted Files	Coverage Δ
...nsensus/tendermint/apps/keymanager/transactions.go	0.00% <0.00%> (-52.95%)	⬇️
go/ias/http/http.go	20.33% <0.00%> (-44.07%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
go/keymanager/api/policy_sgx.go	33.33% <0.00%> (-13.34%)	⬇️
go/runtime/host/sandbox/sandbox.go	71.06% <0.00%> (-11.36%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/consensus/tendermint/abci/state/state.go	61.53% <0.00%> (-7.70%)	⬇️
go/oasis-node/cmd/ias/auth.go	68.96% <0.00%> (-6.90%)	⬇️
...consensus/tendermint/apps/keymanager/keymanager.go	59.34% <0.00%> (-6.60%)	⬇️
go/common/sgx/aesm/aesm.go	63.63% <0.00%> (-6.37%)	⬇️
... and 42 more

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