Failure to install on Windows

Problem

Rocket's Windows CI (Azure Pipelines) started failing between 27 and 4 days ago. We run this command:

curl -sSf -o rustup-init.exe https://win.rustup.rs
rustup-init.exe -y --default-toolchain $(rust_version)

And the output looks like this:

========================== Starting Command Output ===========================
"C:\windows\system32\cmd.exe" /D /E:ON /V:OFF /S /C "CALL "d:\a\_temp\8545808c-7a04-4daf-ad75-cd45a608d93d.cmd""
info: profile set to 'default'
info: default host triple is i686-pc-windows-msvc
error: toolchain 'stable' is not installed
error: caused by: not a directory: 'C:\Users\VssAdministrator\.rustup\toolchains\stable-i686-pc-windows-msvc'
##[error]Cmd.exe exited with code '1'.
Finishing: Install Rust (nightly)

In a previous successful run, this was the output:

========================== Starting Command Output ===========================
"C:\windows\system32\cmd.exe" /D /E:ON /V:OFF /S /C "CALL "D:\a\_temp\60e92b2b-f745-49cf-b841-7987dd621bcc.cmd""
info: profile set to 'default'
info: default host triple is x86_64-pc-windows-msvc
info: updating existing rustup installation

Steps

I have not attempted to reproduce this yet. I'll try locally first, but I would not be surprised if there's some kind of caching going on from a previous installation that will make reproduction difficult.

Analysis

Two things about this seem really odd to me: i686 (maybe related to #2179?) and stable when the requested version was nightly. I don't know if it's trying to find the stable toolchain before or during a self-update, assuming it is trying to do one.

Any hints before I go and try this on bare metal?

Start by examining the Azure Pipelines Windows installation command and compare the failing and successful rustup output, focusing on the unexpected i686 host and stable toolchain. Reproduce the installation behavior if possible and identify the cause; done means the Windows CI installation reliably installs the requested toolchain.