Fix platform.sh failing to detect OS on cachyos

[germanoeich]

CachyOS is based on Arch, so the arch command is not available by default. It does not have the "arch" string in /etc/os-release file either, so platform.sh was failing to detect the architecture, causing the automatic skia download on aseprites build.sh script to fail.

CachyOS os-release file:

❯ cat /etc/os-release
NAME="CachyOS Linux"
PRETTY_NAME="CachyOS"
ID=cachyos
BUILD_ID=rolling
ANSI_COLOR="38;2;23;147;209"
HOME_URL="https://cachyos.org/"
DOCUMENTATION_URL="https://wiki.cachyos.org/"
SUPPORT_URL="https://discuss.cachyos.org/"
BUG_REPORT_URL="https://github.com/cachyos"
PRIVACY_POLICY_URL="https://terms.archlinux.org/docs/privacy-policy/"
LOGO=cachyos

I am not sure if CachyOS is the only impacted distro, so perhaps a more generic "arch" command available -> use it or fallback to uname using command -v arch would be more robust, but since I am not aware of the specifics and dependencies of this file, I didn't want to overreach and introduce a behaviour change.

I agree that my contributions are licensed under the MIT License.
You can find a copy of this license at https://opensource.org/licenses/MIT

[Kaffeine]

The issue should be (and is expected to be) solved on CachyOS side: CachyOS/distribution#177