Use built-in xdg-open on Android

[CrshOverride]

When using opn inside of Termux on Android devices (and Chrome OS), the local version of xdg-open fails because of architecture issues. In these situations xdg-open is available system-wide. This PR updates the cmd to use the system-wide xdg-open on android devices.

[sindresorhus]

the local version of xdg-open fails because of architecture issues

What architecture issues? How is the system-wide xdg-open different from the bundled one?

[CrshOverride]

@sindresorhus My apologies. I failed to look at xdg-open and assumed the behavior was what I was seeing elsewhere. Architecture has nothing to do with the issue. The file layout when using termux on an Android/Chrome OS device is the issue. There is no /bin/sh or /usr/bin/env. Everything lives in /data/data/com.termux/files/usr/bin/sh, etc.

The error I'm seeing is because /bin/sh from the shebang can't be found on my device. Using the xdg-open from the PATH uses a version with the proper shebang for Android/Chrome OS devices.