Build release binaries using PGO

[Jacalz]

Checklist

• [X] I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• [X] 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?

Go 1.20 added Profile Guided Optimization (PGO) support but it was a feature preview and not turned on by default. Go 1.21, which was recently released, improved the support out of feature preview state and now default to -pgo=auto for automatically building with PGO if there is a default.pgo file in the project root. Enabling PGO is, as of 1.21, said to generally result in a 2 to 7% percent performance improvement.

Describe the solution you'd like to see.

Enable PGO once we build release binaries with Go 1.21. This not only requires that fyne-cross-images updates the Go version but also that we can more easily verify that we actually gain any performance.