x86_64-fortanix-unknown-sgx only built on nightly

[jethrogb]

Since the x86_64-fortanix-unknown-sgx target is only available on nightly, ring CI only builds for that target on nightly.

If rust-lang/rust#57659 is accepted, the target will land in beta, which will make it available on stable at 1.33.0. If it is not accepted, it will land in stable at 1.34.0.

[briansmith]

Closing as a duplicate of #779.