Don't force-install xargo
Don't force-install xargo

Previously miri used `cargo install xargo -f` which shouldn't be
necessary anymore since `cargo install` will now upgrade without `-f`.

The only reason I can see to use `-f` is from the cargo docs:

> This is also useful if something has changed on the system that you
> want to rebuild with, such as a newer version of `rustc`.

See the [discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/why.20does.20miri.20require.20xargo/near/214351239).

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