go/worker/common: Properly handle dynamic key manager configuration

[kostko]

Since the runtime can go from having no key manager configured to having
one, the worker node should handle this correctly.

[codecov]

Codecov Report

Merging #4715 (22ee108) into master (cf1783b) will decrease coverage by 0.60%.
The diff coverage is 90.56%.

❗ Current head 22ee108 differs from pull request most recent head eeee071. Consider uploading reports for the commit eeee071 to get more accurate results

@@            Coverage Diff             @@
##           master    #4715      +/-   ##
==========================================
- Coverage   67.47%   66.86%   -0.61%     
==========================================
  Files         439      440       +1     
  Lines       49183    49224      +41     
==========================================
- Hits        33186    32916     -270     
- Misses      11970    12252     +282     
- Partials     4027     4056      +29     

Impacted Files	Coverage Δ
go/worker/common/committee/keymanager.go	89.47% <89.47%> (ø)
go/worker/compute/executor/committee/node.go	71.41% <90.00%> (+0.06%)	⬆️
go/worker/common/committee/node.go	59.38% <100.00%> (-1.48%)	⬇️
go/worker/common/committee/runtime_host.go	50.00% <100.00%> (-8.34%)	⬇️
go/ias/http/http.go	20.33% <0.00%> (-44.07%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
go/keymanager/api/policy_sgx.go	33.33% <0.00%> (-13.34%)	⬇️
go/runtime/host/sgx/epid.go	63.79% <0.00%> (-10.35%)	⬇️
go/runtime/host/sandbox/sandbox.go	66.54% <0.00%> (-10.22%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
... and 54 more

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