oasis-net-runner: Support fixtures without a keymanager

[ptrus]

Creating a fixture without a keymanager fails in oasis-core 22.0, since setting the keymanager binary to an empty path fails with:

Error: root: failed to instantiate fixture: oasis/runtime: failed to read ELF binary: open : no such file or directory

The keymanager deployment binary should probably only be set if if usingKeymanager

oasis-core/go/oasis-net-runner/fixtures/default.go

Lines 129 to 135 in bfe6018

	Deployments: []oasis.DeploymentCfg{
	{
	Binaries: map[node.TEEHardware]string{
	tee: viper.GetString(cfgKeymanagerBinary),
	},
	},
	},