go/runtime/host/sandbox: Fix possible data race

[kostko]

The data race existed because the cancel function that is referenced inside a
goroutine waiting for initialization to complete was unintentionally
overwritten.

[ptrus]

just noticed it as well 😁

[codecov]

Codecov Report

Merging #3024 into master will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3024      +/-   ##
==========================================
+ Coverage   68.45%   68.73%   +0.27%     
==========================================
  Files         367      367              
  Lines       36130    36130              
==========================================
+ Hits        24734    24834     +100     
+ Misses       8233     8148      -85     
+ Partials     3163     3148      -15     

Impacted Files	Coverage Δ
go/runtime/host/sandbox/sandbox.go	77.32% <100.00%> (ø)
go/consensus/tendermint/apps/roothash/query.go	68.42% <0.00%> (-10.53%)	⬇️
...ompute/txnscheduler/algorithm/batching/batching.go	78.66% <0.00%> (-6.67%)	⬇️
go/consensus/tendermint/apps/registry/query.go	80.95% <0.00%> (-4.77%)	⬇️
.../consensus/tendermint/apps/epochtime_mock/state.go	78.72% <0.00%> (-4.26%)	⬇️
.../consensus/tendermint/apps/roothash/state/state.go	68.62% <0.00%> (-3.93%)	⬇️
go/consensus/tendermint/registry/registry.go	68.89% <0.00%> (-3.15%)	⬇️
go/worker/compute/merge/committee/node.go	76.47% <0.00%> (-2.46%)	⬇️
go/consensus/tendermint/abci/state.go	69.06% <0.00%> (-2.16%)	⬇️
go/consensus/tendermint/roothash/roothash.go	63.87% <0.00%> (-2.03%)	⬇️
... and 39 more

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