CI: Update and simplify Windows setup (jpsim#332)

Update the Windows CI to use 5.4.3 instead of 5.4.0.  This update enables switching to gha-setup-swift which simplifies the CI setup for Windows.

Author: compnerd

.github/workflows/swiftpm.yml

@@ -75,24 +75,10 @@ jobs:
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: seanmiddleditch/gha-setup-vsdevenv@master
-      - name: Install swift-5.4
-        run: |
-          Install-Binary -Url "https://swift.org/builds/swift-5.4-release/windows10/swift-5.4-RELEASE/swift-5.4-RELEASE-windows10.exe" -Name "installer.exe" -ArgumentList ("-q")
-      - name: Set Environment Variables
-        run: |
-          echo "SDKROOT=C:\Library\Developer\Platforms\Windows.platform\Developer\SDKs\Windows.sdk" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
-          echo "DEVELOPER_DIR=C:\Library\Developer" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
-      - name: Adjust Paths
-        run: |
-          echo "C:\Library\Swift-development\bin;C:\Library\icu-67\usr\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
-          echo "C:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
-      - name: Install Supporting Files
-        run: |
-          Copy-Item "$env:SDKROOT\usr\share\ucrt.modulemap" -destination "$env:UniversalCRTSdkDir\Include\$env:UCRTVersion\ucrt\module.modulemap"
-          Copy-Item "$env:SDKROOT\usr\share\visualc.modulemap" -destination "$env:VCToolsInstallDir\include\module.modulemap"
-          Copy-Item "$env:SDKROOT\usr\share\visualc.apinotes" -destination "$env:VCToolsInstallDir\include\visualc.apinotes"
-          Copy-Item "$env:SDKROOT\usr\share\winsdk.modulemap" -destination "$env:UniversalCRTSdkDir\Include\$env:UCRTVersion\um\module.modulemap"
+      - uses: compnerd/gha-setup-swift@main
+        with:
+          branch: swift-5.4.3-release
+          tag: 5.4.3-RELEASE
       # Commands to run once connected via SSH:
       #
       # >d: