go/runtime/host/multi: Release lock before calling into runtime

[kostko]

Identified on the Testnet, triggered on client nodes.

[codecov]

Codecov Report

Merging #4538 (a0bb6ad) into master (1850794) will decrease coverage by 0.03%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #4538      +/-   ##
==========================================
- Coverage   67.08%   67.05%   -0.04%     
==========================================
  Files         424      424              
  Lines       47713    47715       +2     
==========================================
- Hits        32010    31994      -16     
- Misses      11800    11803       +3     
- Partials     3903     3918      +15     

Impacted Files	Coverage Δ
go/runtime/host/multi/multi.go	75.00% <75.00%> (+4.10%)	⬆️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
...onsensus/tendermint/apps/beacon/state/state_vrf.go	73.33% <0.00%> (-13.34%)	⬇️
go/consensus/tendermint/apps/staking/votes.go	57.89% <0.00%> (-10.53%)	⬇️
go/worker/storage/committee/utils.go	92.30% <0.00%> (-7.70%)	⬇️
go/worker/keymanager/handler.go	70.00% <0.00%> (-6.67%)	⬇️
go/worker/storage/p2p/sync/client.go	80.64% <0.00%> (-6.46%)	⬇️
...nsensus/tendermint/apps/staking/signing_rewards.go	21.62% <0.00%> (-5.41%)	⬇️
go/worker/beacon/tx_retry.go	90.47% <0.00%> (-4.77%)	⬇️
go/storage/mkvs/lookup.go	75.00% <0.00%> (-4.17%)	⬇️
... and 39 more

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