Add artifact preparation and update upload step

Removed commented-out lines for `MEXLIBCZI_HEADERS` and `MEXLIBCZI_LIBS` in the `cmake` command. Noted potential redundancy with duplicated `cmake --build . --config Release` command.

Added "Prepare artifact" step:
- Uses `bash` shell.
- Creates `artifacts` directory.
- Constructs artifact name using `MEXlibCZI-linux-x64-<git-describe>`.
- Copies `MEXlibCZI.mexa64` to artifact directory.
- Sets `artifactName` and `artifactPath` environment variables.

Updated "Upload artifacts" step to use `actions/upload-artifact@v4` and upload using the set environment variables.

.github/workflows/build_linuxx64.yml

@@ -25,4 +25,18 @@ jobs:
           cd build
           #cmake .. -DCMAKE_BUILD_TYPE=Release -DMEXLIBCZI_HEADERS=$MATLAB_HEADER_PATH -DMEXLIBCZI_LIBS=$MATLAB_LIB_PATH
           cmake .. -DCMAKE_BUILD_TYPE=Release 
-          cmake --build . --config Release
+          cmake --build . --config Release
+      - name: Prepare artifact
+        shell: bash
+        run: |
+          mkdir -p artifacts
+          name="MEXlibCZI-linux-x64-$(git describe --always)"
+          mkdir -p artifacts/${name}
+          cp build/MEXlibCZI/MEXlibCZI.mexa64 artifacts/${name}/
+          echo "artifactName=${name}" >> "$GITHUB_ENV"
+          echo "artifactPath=artifacts/${name}" >> "$GITHUB_ENV"
+      - name: Upload artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          path: ${{ env.artifactPath }}/
+          name: ${{ env.artifactName }}