[Build] Set up single upload of test coverage

.github/workflows/rust.yml

@@ -44,6 +44,7 @@ jobs:
 
       - name: Run cargo-tarpaulin
         uses: actions-rs/tarpaulin@v0.1
+        if: ${{ matrix.rust == 'stable' }}
         with:
           version: '0.15.0'
           out-type: Xml
@@ -52,11 +53,13 @@ jobs:
 
       - name: Upload to codecov.io
         uses: codecov/codecov-action@v1.0.2
+        if: ${{ matrix.rust == 'stable' }}
         with:
           token: ${{ secrets.CODECOV_SECRET }}
 
       - name: Archive code coverage results
         uses: actions/upload-artifact@v1
+        if: ${{ matrix.rust == 'stable' }}
         with:
           name: code-coverage-report
           path: cobertura.xml