Make @nix json structured build log parsing warn instead of fail

[roberth]

Motivation

A build may log with @nix something without the intent to write Nix structured logs, resulting in an obscure error. This makes it easy to understand and continues the build.

Example:

echo '@nix ]['

bad JSON log message from builder: [json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
@nix ][

Context

Cherry-picked from https://gerrit.lix.systems/c/lix/+/2057 Author @lheckemann

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[roberth]

Interestingly, the post-build-hook does not have @nix parsing logic, so that's what that's not in the diff.