Use SSH_KEY as TagBot secret

[simonbyrne]

Follow on from #466

[codecov]

Codecov Report

Base: 76.30% // Head: 78.01% // Increases project coverage by +1.70% 🎉

Coverage data is based on head (060bd0e) compared to base (ca8b993).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #467      +/-   ##
==========================================
+ Coverage   76.30%   78.01%   +1.70%     
==========================================
  Files          23       23              
  Lines        1570     1610      +40     
==========================================
+ Hits         1198     1256      +58     
+ Misses        372      354      -18     

Impacted Files	Coverage Δ
src/methods.jl	67.82% <0.00%> (+0.34%)	⬆️
src/render.jl	88.57% <0.00%> (+1.42%)	⬆️
src/setup.jl	76.13% <0.00%> (+4.70%)	⬆️
src/eventloop.jl	68.51% <0.00%> (+14.81%)	⬆️
src/io.jl	71.76% <0.00%> (+18.19%)	⬆️

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