fix: download and patch fw binaries on nixos

[szymonlesisz]

i had it stashed for a while now and used it only locally

im running user-env natively on nixos and having some issues since the configuration changed

1. dowloading binaries produces copies instead of override existing file. Wtih this fix it will skip dowloading
2. patching binaries does not work, the process doesnt throw any error but patched binary cannot be started

[szymonlesisz]

i believe that failing test is not related to my changes

[grdddj]

i believe that failing test is not related to my changes

Right, the failure is known, and it comes from trezorlib - I am attempting to fix it here - trezor/trezor-firmware#4664

[grdddj]

Useful things, thanks!