fix(maker-flatpak): fix flatpakArch for arm64

[pano9000]

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Summarize your changes:

Hi,

this PR is a fix for the issue described in #3838
It fixes it by adding translating the "arm64" architecture string from electron-forge, into "aarch64", which is what Flatpak is actually expecting.
The same thing was done for "x64" and "x86_64" as well.

closes #3838

[erickzhao]

Seems reasonable to me.