Fixes coveralls coverage report

With PR btcsuite#1785 multiple coverage reports are sent to coveralls. This
makes coveralls only consider the last report sent. This fix treat all
reports has parallel reports so they can all be compiled later by coveralls.

.github/workflows/go.yml

@@ -42,21 +42,34 @@ jobs:
         uses: shogo82148/actions-goveralls@v1
         with:
           path-to-profile: coverage.txt
+          flag-name: btcd
+          parallel: true
 
       - name: Send btcec
         uses: shogo82148/actions-goveralls@v1
         with:
           path-to-profile: btcec/coverage.txt
+          flag-name: btcec
+          parallel: true
 
       - name: Send btcutil coverage
         uses: shogo82148/actions-goveralls@v1
         with:
           path-to-profile: btcutil/coverage.txt
+          flag-name: btcutil
+          parallel: true
 
       - name: Send btcutil coverage for psbt package
         uses: shogo82148/actions-goveralls@v1
         with:
           path-to-profile: btcutil/psbt/coverage.txt
+          flag-name: btcutilpsbt
+          parallel: true
+
+      - name: Notify coveralls all reports sent
+        uses: shogo82148/actions-goveralls@v1
+        with:
+          parallel-finished: true
 
   test-race:
     name: Unit race