Hide beta builds after last rust release

[javierpedreira]

Rust Beta builds are failing because of this.
rust-lang/rust#59411
Until they fix it we'll allow Beta fails

[kittsville]

The discussion in the linked thread suggests using Xenial should be enough to avoid build failures, so we don't need to allow beta failures.

[nathankleyn]

@kittsville Hah! Crossover! @javierpedreira were literally just talking about this — have added xenial as a test and then will remove the allowed failure on beta if it's green. 🤞

[09:13] Nathan Kleyn
funnily enough this is the first time i’ve ever got a breakage on a build from a beta rust
so as soon as it’s green again, we should probably renable it
because beta gets promoted to stable and our build will just use it

[nathankleyn]

So now we know beta works on Xenia’s, let’s make it required again to prevent problems with automatic upgrades of stable.

[kittsville]

LGTM

Changes implemented as requested