Switch CI back to nightly channel (#2014)

* Switch CI back to nightly channel

I think all upstream issues are now fixed so we should be good to switch
back to nightly from our previously pinned version.

* Fix doc warnings

cranelift/codegen/src/ir/constant.rs

@@ -166,7 +166,7 @@ pub type ConstantOffset = u32;
 /// function body); because the function is not yet compiled when constants are inserted,
 /// [`set_offset`](crate::ir::ConstantPool::set_offset) must be called once a constant's offset
 /// from the beginning of the function is known (see
-/// [`relaxation.rs`](crate::binemit::relaxation)).
+/// `relaxation` in `relaxation.rs`).
 #[derive(Clone)]
 pub struct ConstantPoolEntry {
     data: ConstantData,

cranelift/codegen/src/isa/aarch64/abi.rs

@@ -23,7 +23,7 @@
 //! being adjusted to set up a call, we implement a "nominal SP" tracking
 //! feature by which a fixup (distance between actual SP and a "nominal" SP) is
 //! known at each instruction. See the documentation for
-//! [MemArg::NominalSPOffset] for more on this.
+//! `MemArg::NominalSPOffset` for more on this.
 //!
 //! The stack looks like:
 //!