[ci] Generate only XML coverage data

That is the one we are uploading, and HTML coverage seems broken on
macOS.
diff --git a/.github/workflows/linux-ci.yml b/.github/workflows/linux-ci.yml
index 88b1f59..407d214 100644
--- a/.github/workflows/linux-ci.yml
+++ b/.github/workflows/linux-ci.yml
@@ -47,7 +47,7 @@
     #     fail-on-alert: true
 
     - name: cov
-      run: ninja -Cbuild coverage
+      run: ninja -Cbuild coverage-xml
     - uses: codecov/codecov-action@v1
       with:
         file: build/meson-logs/coverage.xml
diff --git a/.github/workflows/macos-ci.yml b/.github/workflows/macos-ci.yml
index 0ee1075..84e54df 100644
--- a/.github/workflows/macos-ci.yml
+++ b/.github/workflows/macos-ci.yml
@@ -21,7 +21,7 @@
       run: meson test --print-errorlogs -Cbuild
 
     - name: cov
-      run: ninja -Cbuild coverage
+      run: ninja -Cbuild coverage-xml
     - uses: codecov/codecov-action@v1
       with:
         file: build/meson-logs/coverage.xml