config: apply environment overrides before unmarshal

[dlanov]

Fixes #833

GNMIC_* environment variables were merged into Viper only after the
configuration had already been unmarshaled into Config. As a result,
nested collector configuration such as
GNMIC_CLUSTERING_LOCKER_ADDRESS was present in FileConfig but was
not reflected in the configuration object stored for collector mode.

Merge environment values before unmarshaling so both global and nested
configuration fields receive environment overrides.

Add a regression test verifying that:

• GNMIC_CLUSTER_NAME populates the global cluster name
• GNMIC_CLUSTERING_LOCKER_ADDRESS overrides the value from the configuration file
• unrelated clustering values from the file remain intact

Testing:

• go test ./pkg/config -run '^TestLoadEnvironmentOverridesConfig$' -count=1 -v
• go test ./... -count=1
• go vet ./...