chore: use go1.15 in bertytech/protoc:26

[moul]

Related with daixiang0/gci#23
Addresses #2771

[codecov]

Codecov Report

Merging #2773 (b066e3c) into master (1218c3b) will decrease coverage by 1.41%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2773      +/-   ##
==========================================
- Coverage   27.75%   26.33%   -1.42%     
==========================================
  Files         284      306      +22     
  Lines       16534    17803    +1269     
  Branches     1245     1245              
==========================================
+ Hits         4589     4689     +100     
- Misses      11167    12327    +1160     
- Partials      778      787       +9     

Flag	Coverage Δ
go.unittests	31.05% <ø> (-2.49%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
go/pkg/bertyprotocol/api_group.go	15.20% <0.00%> (-24.39%)	⬇️
go/pkg/bertymessenger/service.go	34.63% <0.00%> (-7.29%)	⬇️
go/internal/initutil/ipfs.go	48.39% <0.00%> (-2.39%)	⬇️
...protocol/orbitdb_signed_entry_identity_provider.go	56.81% <0.00%> (-2.28%)	⬇️
go/internal/initutil/datastore.go	26.41% <0.00%> (-2.16%)	⬇️
go/cmd/berty/banner.go	39.28% <0.00%> (-1.46%)	⬇️
go/cmd/berty/groupinit.go	30.43% <0.00%> (-1.39%)	⬇️
go/cmd/berty/peers.go	25.80% <0.00%> (-0.87%)	⬇️
go/cmd/berty/shareinvite.go	29.72% <0.00%> (-0.83%)	⬇️
go/cmd/berty/daemon.go	20.58% <0.00%> (-0.63%)	⬇️
... and 48 more

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[github-actions]

🎉 This PR is included in version 2.214.1 🎉

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