[Windows] Cleanup various scripts (#8942)

* Use Resolve-GithubReleaseAssetUrl more widely

* Add the Get-ChecksumFromUrl function

* Sort exported functions and add docs

* Remove alias and fix typo

* Fix kind checksum url and syntax

* Fix checksums url for gh cli and msys2

* [Windows] Cleanup various scripts

* Add spaces after type specifications

* Rename the Take-Part function

images/windows/scripts/build/Install-PowershellCore.ps1

@@ -16,10 +16,11 @@ try {
     $release = $metadata.LTSReleaseTag[0] -replace '^v'
     $downloadUrl = "https://github.com/PowerShell/PowerShell/releases/download/v${release}/PowerShell-${release}-win-x64.msi"
 
-    $hashUrl = "https://github.com/PowerShell/PowerShell/releases/download/v${release}/hashes.sha256"
-    $expectedSHA256Sum = (Invoke-RestMethod -Uri $hashURL).ToString().Split("`n").Where({ $_ -ilike "*PowerShell-${Release}-win-x64.msi*" }).Split(' ')[0]
-
-    Install-Binary -Url $downloadUrl -ExpectedSHA256Sum $expectedSHA256Sum
+    $installerName = Split-Path $downloadUrl -Leaf
+    $externalHash = Get-ChecksumFromUrl -Type "SHA256" `
+        -Url ($downloadUrl -replace $installerName, "hashes.sha256") `
+        -FileName $installerName
+    Install-Binary -Url $downloadUrl -ExpectedSHA256Sum $externalHash
 } finally {
     # Restore original value
     [Net.ServicePointManager]::SecurityProtocol = $originalValue
@@ -30,6 +31,6 @@ try {
 # While the update check happens during the first session in a given 24-hour period, for performance reasons,
 # the notification will only be shown on the start of subsequent sessions.
 # Also for performance reasons, the check will not start until at least 3 seconds after the session begins.
-[System.Environment]::SetEnvironmentVariable("POWERSHELL_UPDATECHECK", "Off", [System.EnvironmentVariableTarget]::Machine)
+[Environment]::SetEnvironmentVariable("POWERSHELL_UPDATECHECK", "Off", "Machine")
 
 Invoke-PesterTests -TestFile "Tools" -TestName "PowerShell Core"