Speed up PR CI: use RunTests target, add NuGet cache, upgrade actions to v4 (#1656)

- Change build-windows from '-t All' to '-t RunTests': skips doc generation
  and pack steps that aren't needed for PR validation (~4-5 min saved)
- Remove redundant 'Build (Debug)' step from both jobs (~2 min combined)
- Add NuGet package caching via actions/cache@v4 (~30-40s saved on cache hits)
- Upgrade actions/checkout v1 → v4 and actions/setup-dotnet v1 → v4

These changes should reduce total PR CI time from ~12 min to ~6-7 min.

Closes #1649

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

.github/workflows/pull-requests.yml

@@ -11,36 +11,46 @@ jobs:
 
     runs-on: windows-latest
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v4
     - name: Setup .NET Core 8
-      uses: actions/setup-dotnet@v1
+      uses: actions/setup-dotnet@v4
       with:
         dotnet-version: 8.0.400
+    - name: Cache NuGet packages
+      uses: actions/cache@v4
+      with:
+        path: ~/.nuget/packages
+        key: ${{ runner.os }}-nuget-${{ hashFiles('**/paket.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-nuget-
     - name: Restore .NET local tools
       run: dotnet tool restore
     - name: Restore packages
       run: dotnet paket restore
     - name: Build and test (Release)
       env:
         FAKE_DETAILED_ERRORS: true
-      run: dotnet run --project build/build.fsproj -- -t All
-    - name: Build (Debug)
-      run: dotnet build -c Debug -v n
+      run: dotnet run --project build/build.fsproj -- -t RunTests
 
   build-ubuntu:
 
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v4
     - name: Setup .NET Core 8
-      uses: actions/setup-dotnet@v1
+      uses: actions/setup-dotnet@v4
       with:
         dotnet-version: 8.0.400
+    - name: Cache NuGet packages
+      uses: actions/cache@v4
+      with:
+        path: ~/.nuget/packages
+        key: ${{ runner.os }}-nuget-${{ hashFiles('**/paket.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-nuget-
     - name: Restore .NET local tools
       run: dotnet tool restore
     - name: Restore packages
       run: dotnet paket restore
     - name: Build and test
       run: dotnet run --project build/build.fsproj -- -t RunTests
-    - name: Build (Debug)
-      run: dotnet build -c Debug -v n