Add x86_64 to Windows installer file name

The current Windows installer file name convention does not indicate whether it is 32-bit or 64-bit. 64-bit is implicit because it is currently (3.0.0 being current) the only architecture supported on Windows. This becomes a problem when #281539: MuseScore 3 on Windows 32-bit is not available is resolved. Current name:

• MuseScore-3.0.0.msi

Change to:

• MuseScore-3.0.0-x86_64.msi

This will make the architecture clear and leave room for 32-bit MuseScore beside it:

• MuseScore-3.0.0-x86.msi

And follow the same naming convention as Linux,

• MuseScore-3.0.0-x86_64.AppImage

Alltogether, that would leave us with

• MuseScore-3.0.0-x86.msi
• MuseScore-3.0.0-x86_64.msi
• MuseScore-3.0.0-x86_64.AppImage

The workaround / alternative would be to do nothing, but leave a very inconsistent naming convention, clashing with both 32-bit Windows and Linux

• MuseScore-3.0.0-x86.msi
• MuseScore-3.0.0.msi
• MuseScore-3.0.0-x86_64.AppImage

Note: macOS does not need the architecture since Mac hardware has been 64-bit since 2009. Still, it would make it super-consistent with all the other file names:

• MuseScore-3.0.0.dmg

To

• MuseScore-3.0.0-x86_64.dmg