[infra] Change language attribute from "cpp" to "c++" in coverage job…

… runner.

Should've done this in #3477. Good news is that nothing failed, the coverage job was just skipped for pretty much all projects.

infra/gcb/build_and_run_coverage.py

@@ -36,7 +36,7 @@
 UPLOAD_URL_FORMAT = 'gs://' + COVERAGE_BUCKET_NAME + '/{project}/{type}/{date}'
 
 # Languages from project.yaml that have code coverage support.
-LANGUAGES_WITH_COVERAGE_SUPPORT = ['c', 'cpp']
+LANGUAGES_WITH_COVERAGE_SUPPORT = ['c', 'c++']
 
 
 def skip_build(message):