replace arch with uname -m

[bwalex]

The arch command isn't particularly well standardized compared to uname -m. For example, coreutils on Archlinux doesn't provide it, and it's generally not recommended to rely on it.

See e.g. https://www.gnu.org/software/coreutils/manual/html_node/arch-invocation.html:

arch is not installed by default, so portable scripts should not rely on its existence.

[guysoft]

Thanks for your contribution