Como posso corrigir esse erro com a atualização:
Err:26 https://storage.googleapis.com/download.dartlang.org/linux/debian stable Release.gpg
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E88979FB9B30ACF2
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W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://storage.googleapis.com/download.dartlang.org/linux/debian stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E88979FB9B30ACF2
W: Failed to fetch https://storage.googleapis.com/download.dartlang.org/linux/debian/dists/stable/Release.gpg The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E88979FB9B30ACF2
W: Some index files failed to download. They have been ignored, or old ones used instead.