Fix handling of toolchain-specific Windows ARM32 compiler. (#2255)

- Ensure that compiler zip and expanded directory are resolved relative to
  the repo root, not current directory.

  This is needed because the azureiotedge-diagnostics build script changes
  the current directory when it switches from building amd64 to arm32.

- Ensure that compiler zip and expanded directory contain the toolchain name.

  Previously the existing expanded directory would be used regardless of
  which toolchain zip was expanded to create it.

  This is also needed because the build agent is shared for older branches that
  do not use the toolchain file, so their zip and expanded directories
  must not conflict.

edgelet/build/windows/util.ps1

@@ -10,7 +10,9 @@ function Test-RustUp
 
 function GetPrivateRustPath
 {
-    Join-Path -Path (Get-IotEdgeFolder) -ChildPath 'rust-windows-arm/rust-windows-arm/bin/'
+    $toolchain = Get-Content -Encoding UTF8 (Join-Path -Path (Get-EdgeletFolder) -ChildPath 'rust-toolchain')
+    $targetPath = Join-Path -Path (Get-IotEdgeFolder) -ChildPath "rust-windows-arm-$toolchain"
+    Join-Path -Path $targetPath -ChildPath 'rust-windows-arm/bin/'
 }
 
 function Get-CargoCommand
@@ -54,9 +56,10 @@ function Assert-Rust
 
     $ErrorActionPreference = 'Continue'
 
+    $toolchain = Get-Content -Encoding UTF8 (Join-Path -Path (Get-EdgeletFolder) -ChildPath 'rust-toolchain')
+
     if ($Arm) {
-        if (-not (Test-Path 'rust-windows-arm')) {
-            # if the folder rust-windows-arm exists, we assume the private rust compiler for arm is installed
+        if (-not (Test-Path (GetPrivateRustPath))) {
             InstallWinArmPrivateRustCompiler
         }
     }
@@ -77,7 +80,6 @@ function Assert-Rust
             }
         }
 
-        $toolchain = Get-Content -Encoding UTF8 (Join-Path -Path (Get-EdgeletFolder) -ChildPath 'rust-toolchain')
         Write-Host "Installing / updating $toolchain toolchain"
         rustup update $toolchain
         if ($LastExitCode)
@@ -90,15 +92,25 @@ function Assert-Rust
 }
 
 function InstallWinArmPrivateRustCompiler {
+    $ProgressPreference = 'SilentlyContinue'
+
     $toolchain = Get-Content -Encoding UTF8 (Join-Path -Path (Get-EdgeletFolder) -ChildPath 'rust-toolchain')
-    $link = "https://edgebuild.blob.core.windows.net/iotedge-win-arm32v7-tools/rust-windows-arm-$toolchain.zip"
 
-    Write-Host "Downloading $link"
-    $ProgressPreference = 'SilentlyContinue'
-    Invoke-WebRequest $link -OutFile 'rust-windows-arm.zip' -UseBasicParsing
+    $downloadPath = (Join-Path -Path (Get-IotEdgeFolder) -ChildPath "rust-windows-arm-$toolchain.zip")
+    if (-not (Test-Path $downloadPath)) {
+        $link = "https://edgebuild.blob.core.windows.net/iotedge-win-arm32v7-tools/rust-windows-arm-$toolchain.zip"
+
+        Write-Host "Downloading $link to $downloadPath"
+        Invoke-WebRequest $link -OutFile $downloadPath -UseBasicParsing
+    }
+
+    $targetPath = Join-Path -Path (Get-IotEdgeFolder) -ChildPath "rust-windows-arm-$toolchain"
+    Write-Host "Extracting $downloadPath to $targetPath"
+    if (Test-Path $targetPath) {
+        Remove-Item -Recurse -Force $targetPath
+    }
+    Expand-Archive -Path $downloadPath -DestinationPath $targetPath
 
-    Write-Host "Extracting $link"
-    Expand-Archive -Path 'rust-windows-arm.zip' -DestinationPath 'rust-windows-arm'
     $ProgressPreference = 'Stop'
 }