libutil: Fix i686-linux build on clangStdenv

[xokdvium]

Motivation

Clang refused to do a narrowing conversion in an initializer list:

 local-keys.cc:56:90: note: insert an explicit cast to silence this issue
return name + ":" + base64::encode(std::as_bytes(std::span<const unsigned char>{sig, sigLen}));
                                                                                    ^~~~~~
                                                                                    static_cast<size_type>( )

Context

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