ci: add go caching

[moul]

actions/setup-go requires actions/checkout to read the go.sum file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.34%. Comparing base (7ef606c) to head (cd16cfd).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3091   +/-   ##
=======================================
  Coverage   63.34%   63.34%           
=======================================
  Files         548      548           
  Lines       78680    78680           
=======================================
  Hits        49836    49836           
+ Misses      25482    25481    -1     
- Partials     3362     3363    +1     

Flag	Coverage Δ
contribs/gnodev	60.54% <ø> (-0.63%)	⬇️
contribs/gnofaucet	15.77% <ø> (+0.94%)	⬆️
gno.land	67.13% <ø> (ø)
gnovm	67.90% <ø> (ø)
misc/genstd	79.72% <ø> (ø)
tm2	62.31% <ø> (-0.04%)	⬇️

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