Install missing rustup in windows-11-arm jobs

This works around the combination of these two conditions, which
individually wouldn't be a problem but are a problem together:

- The newly available `windows-11-arm` runner does not currently
  have `rustup` installed on it.

- Although the `dtolnay/rust-toolchain` action will attempt to
  install `rustup` when it is absent on Unix-like systems, it does
  not attempt to do so on Windows.

(If the latter condition is fixed, however, then depending on *how*
it is fixed, it might still be necessary to install `rustup`
ourselves. Specifically, it is easy to accidentally download a
`rustup-init.exe` for x86-64 even when on ARM64/AArch64 Windows.)

The new step here, added for both of the `windows-11-arm` jobs, is
intended to be similar in its effect to these commands that
`dtolnay/rustup-init` runs on Unix-like systems:
https://github.com/actions-rust-lang/setup-rust-toolchain/blob/9399c7bb15d4c7d47b27263d024f0a4978346ba4/action.yml#L136-L139

Note that this is not quite equivalent. It is intentionally less
versatile in two ways:

- It hard-codes the AArch64 `rustup-init.exe` URL, to ensure that
  it is selected, rather than not finding anything or wrongly
  getting a URL for a Unix-like system or for x86-64 Windows.

- No attempt is made to respect a preexisting `CARGO_HOME`
  environment variable (since we do not set one in these jobs).

(This uses the style `$Env:varname` rather than `$env:varname` in
PowerShell because the former seems to be much more common in the
Microsoft documentation, and changes one preexisting occurrence
of `$env` to `$Env` for stylistic consistency.)

Author: EliahKagan

.github/workflows/ci.yml

@@ -217,6 +217,19 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
+      - name: Install rustup
+        if: matrix.os == 'windows-11-arm'
+        run: |
+          if (Get-Command rustup -ErrorAction SilentlyContinue) {
+            Write-Output '::warning:: The runner has rustup. Consider removing this step.'
+          } else {
+            curl --proto '=https' --tlsv1.2 --retry 10 --retry-connrefused -fsSL `
+              --output rustup-init.exe `
+              https://static.rust-lang.org/rustup/dist/aarch64-pc-windows-msvc/rustup-init.exe
+            .\rustup-init.exe --default-toolchain none -y
+            Remove-Item rustup-init.exe
+            Add-Content -Value "PATH=$Env:USERPROFILE\.cargo\bin;$Env:PATH" -Path $Env:GITHUB_ENV
+          }
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - name: cargo check default features
@@ -241,6 +254,19 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
+      - name: Install rustup
+        if: matrix.os == 'windows-11-arm'
+        run: |
+          if (Get-Command rustup -ErrorAction SilentlyContinue) {
+            Write-Output '::warning:: The runner has rustup. Consider removing this step.'
+          } else {
+            curl --proto '=https' --tlsv1.2 --retry 10 --retry-connrefused -fsSL `
+              --output rustup-init.exe `
+              https://static.rust-lang.org/rustup/dist/aarch64-pc-windows-msvc/rustup-init.exe
+            .\rustup-init.exe --default-toolchain none -y
+            Remove-Item rustup-init.exe
+            Add-Content -Value "PATH=$Env:USERPROFILE\.cargo\bin;$Env:PATH" -Path $Env:GITHUB_ENV
+          }
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - uses: taiki-e/install-action@v2
@@ -344,7 +370,7 @@ jobs:
         with:
           tool: nextest
       - name: Test data structure sizes (nextest)
-        run: cargo nextest run --target $env:TARGET --workspace --no-fail-fast size
+        run: cargo nextest run --target $Env:TARGET --workspace --no-fail-fast size
       - name: Doctest
         run: cargo test --workspace --doc --no-fail-fast