Bump Go to 1.15.5

[tjanez]

This fixes a recently disclosed security vulnerability in the Go runtime library.

[codecov]

Codecov Report

Merging #3512 (73a3acb) into master (80ac592) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3512      +/-   ##
==========================================
- Coverage   66.54%   66.43%   -0.12%     
==========================================
  Files         373      373              
  Lines       33956    33956              
==========================================
- Hits        22596    22558      -38     
- Misses       8089     8120      +31     
- Partials     3271     3278       +7     

Impacted Files	Coverage Δ
go/common/sgx/aesm/aesm.go	62.61% <0.00%> (-6.55%)	⬇️
...consensus/tendermint/apps/roothash/transactions.go	50.00% <0.00%> (-6.04%)	⬇️
go/storage/mkvs/lookup.go	72.04% <0.00%> (-4.31%)	⬇️
go/consensus/tendermint/full/services.go	81.35% <0.00%> (-4.24%)	⬇️
go/worker/keymanager/watcher.go	60.00% <0.00%> (-3.34%)	⬇️
.../consensus/tendermint/apps/roothash/state/state.go	74.28% <0.00%> (-2.86%)	⬇️
go/keymanager/client/client.go	78.94% <0.00%> (-2.64%)	⬇️
go/runtime/client/watcher.go	62.96% <0.00%> (-2.23%)	⬇️
go/worker/storage/committee/checkpoint_sync.go	72.76% <0.00%> (-1.95%)	⬇️
go/registry/api/api.go	41.87% <0.00%> (-1.09%)	⬇️
... and 17 more

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