github: Explicitly install ninja where needed in Windows jobs

Ninja isn't documented as one of the tools provided by the CI runner
images, but it used to be available implicitly installed before
in the windows-2022 images. Since the 20230918.1.0 image update,
it no longer is available out of the box; see
https://github.com/actions/runner-images/issues/8343.

.github/workflows/build.yml

@@ -342,6 +342,9 @@ jobs:
           - x86_64
           - i686
     steps:
+      - name: Install dependencies
+        run: |
+          choco install ninja
       - uses: actions/download-artifact@v3
         with:
           name: windows-ucrt-${{matrix.arch}}-toolchain
@@ -397,6 +400,9 @@ jobs:
           - { arch: i686, asmflag: }
           - { arch: x86_64, asmflag: -m64 }
     steps:
+      - name: Install dependencies
+        run: |
+          choco install ninja
       - uses: actions/download-artifact@v3
         with:
           name: windows-ucrt-${{matrix.arch}}-toolchain
@@ -458,6 +464,9 @@ jobs:
           - x86_64
           - i686
     steps:
+      - name: Install dependencies
+        run: |
+          choco install ninja
       - uses: actions/download-artifact@v3
         with:
           name: windows-ucrt-${{matrix.arch}}-toolchain

.github/workflows/test-libcxx.yml

@@ -48,6 +48,9 @@ jobs:
           - x86_64
           - i686
     steps:
+      - name: Install dependencies
+        run: |
+          choco install ninja
       - name: Download toolchain
         uses: dawidd6/action-download-artifact@v2
         with:

.github/workflows/test-llvm.yml

@@ -42,6 +42,9 @@ jobs:
     needs: [prepare]
     runs-on: windows-latest
     steps:
+      - name: Install dependencies
+        run: |
+          choco install ninja
       - name: Download toolchain
         uses: dawidd6/action-download-artifact@v2
         with: