fix(ssvsigner): linter and tests

[nkryuchkov]

#2028 has been merged with broken linter and tests, it likely happened in one of the latest commits. The reason we didn't notice this is that the ssvsigner subfolder has its own go.mod, so it's not covered by the CI checks (similarly to the e2e subfolder)

It would be really great to have linter & tests for the ssv-signer in CI but it might be tricky: golangci/golangci-lint#828

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 47.7%. Comparing base (47ea4bc) to head (f0f0414).
Report is 2 commits behind head on stage.

Files with missing lines	Patch %	Lines
cli/operator/node.go	0.0%	1 Missing ⚠️

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