Consider setting GOAMD64=v2 for release builds

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

It is worth investigating if we should raise the value of GOAMD64 for released binaries, both here on GitHub and Flathub. There might be performance to be gained from using a more modern set of instructions.

Describe the solution you'd like to see.

Investigate if performance is worth it and if it wins over lowered device support. v2 is a very sensible requirement with very few modern devices not supporting it.

This should not be set in the Makefile as distributions will want to do so themselves if applicable.