Xonsh activation doesn't work

[LoicGrobol]

Following #1801, the Xonsh activation doen't work anymore: after activating with "execx($(mise activate xonsh)), any call to mise fails with No such file or directory: 'command'.

The issue is at https://github.com/yggdr/mise/blob/82a7768f97c0ef35dd7b4776ab6a321ec56c9835/src/shell/xonsh.rs#L59, which tries to call the command builtin, which simply doesn't exist in Xonsh, contrarily to bash and fish. It might be good if Xonsh added it but in the meantime, mise activation is simply not usable in Xonsh.