Copy all coverage.html files into artifacts directory (#56)

* Copy all coverage.html files into artifacts directory

This allows archiving/inspection of coverage for all of the submodules
in opentelemetry-go-contrib.

* Fall back to tar with a file list.  cpio missing fromm docker image.

* fixup! Fall back to tar with a file list.  cpio missing fromm docker image.

.circleci/config.yml

@@ -19,7 +19,8 @@ jobs:
           name: "Precommit and Coverage Report"
           command: |
             make ci
-            mv coverage.html $TEST_RESULTS/
+            find . -name 'coverage.html' > "${TEST_RESULTS}/coverage.lst"
+            tar -n -cf - -T "${TEST_RESULTS}/coverage.lst" | tar -C "${TEST_RESULTS}" -xvf -
 
       - save_cache:
           key: go-pkg-mod-{{ checksum "go.sum" }}