Enable development on macOS without docker

[nhynes]

This PR:

• adds conditional compilation to the oasis-core-runtime-loader
• allows setting the provisioner through oasis-net-runner (to unconfined)
• allows running a net without a keymanager since simple-keymanager isn't fun to compile with musl

[codecov]

Codecov Report

Merging #4024 (5eff0c8) into master (bbc6e9d) will increase coverage by 0.18%.
The diff coverage is n/a.

❗ Current head 5eff0c8 differs from pull request most recent head a082cd3. Consider uploading reports for the commit a082cd3 to get more accurate results

@@            Coverage Diff             @@
##           master    #4024      +/-   ##
==========================================
+ Coverage   66.96%   67.15%   +0.18%     
==========================================
  Files         410      410              
  Lines       42183    42183              
==========================================
+ Hits        28249    28329      +80     
+ Misses       9938     9878      -60     
+ Partials     3996     3976      -20     

Impacted Files	Coverage Δ
go/common/sgx/common.go	58.53% <0.00%> (-4.88%)	⬇️
...consensus/tendermint/apps/roothash/transactions.go	66.19% <0.00%> (-3.34%)	⬇️
go/worker/storage/service_external.go	54.28% <0.00%> (-2.86%)	⬇️
go/runtime/nodes/grpc/grpc.go	71.93% <0.00%> (-1.98%)	⬇️
.../consensus/tendermint/apps/roothash/state/state.go	68.64% <0.00%> (-1.70%)	⬇️
go/oasis-node/cmd/stake/delegations.go	79.24% <0.00%> (-0.95%)	⬇️
go/consensus/tendermint/full/services.go	80.00% <0.00%> (ø)
go/consensus/tendermint/roothash/roothash.go	67.40% <0.00%> (ø)
...o/consensus/tendermint/apps/beacon/backend_pvss.go	60.00% <0.00%> (+0.43%)	⬆️
go/worker/common/p2p/peermgmt.go	81.50% <0.00%> (+0.57%)	⬆️
... and 17 more

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