ci: bump actions/download-artifact from v6 to v8

We use `actions/download-artifact` to pass build artifacts between
the "windows-build" / "vs-build" / "windows-meson-build" jobs and
their corresponding test jobs. All callers are currently on v6;
bumping to v8 keeps this action in lockstep with the
`upload-artifact` bump in the previous commit.

What v7 and v8 change:

- v7 switches the default runtime from Node.js 20 to Node.js 24 (v6
  had preliminary Node 24 support but still defaulted to Node 20).
  This is the main motivation: it gets us off the deprecated runtime.
- v8 makes three further changes:
  * The package is converted to ESM (invisible to workflow authors).
  * The action now checks the `Content-Type` header before
    attempting to unzip a download, so that directly-uploaded
    (unzipped) artifacts from `upload-artifact` v7 are downloaded
    correctly.
  * The `digest-mismatch` behaviour is changed from warn-and-
    continue to a hard failure by default.

Risk analysis: defaulting hash-mismatch to a hard failure is
strictly safer than the previous warn-and-continue behaviour -- a
mismatch points to real corruption or tampering and should stop the
run. We download archives that the same workflow just uploaded, on
the same runner fleet, so false positives are not expected. Our
usage is limited to the `name` and `path` inputs, which are
unchanged between v6 and v8, so the diff is purely the `@vN`
identifier.

See also:

- Release notes: https://github.com/actions/download-artifact/releases
- Compare: actions/download-artifact@v6...v8

Originally-authored-by: dependabot[bot] <support@github.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/main.yml

@@ -140,7 +140,7 @@ jobs:
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
     - name: download tracked files and build artifacts
-      uses: actions/download-artifact@v6
+      uses: actions/download-artifact@v8
       with:
         name: windows-artifacts
         path: ${{github.workspace}}
@@ -226,7 +226,7 @@ jobs:
     steps:
     - uses: git-for-windows/setup-git-for-windows-sdk@v1
     - name: download tracked files and build artifacts
-      uses: actions/download-artifact@v6
+      uses: actions/download-artifact@v8
       with:
         name: vs-artifacts
         path: ${{github.workspace}}
@@ -292,7 +292,7 @@ jobs:
       shell: pwsh
       run: pip install meson ninja
     - name: Download build artifacts
-      uses: actions/download-artifact@v6
+      uses: actions/download-artifact@v8
       with:
         name: windows-meson-artifacts
         path: build