Fix failing coverage reporting

[kentzeng12]

Our pipeline tests are running really slowly on buildkite, example: https://buildkite.com/uberopensource/m3-monorepo-ci-uber-oss/builds/679.

This is due to coverage reporting being broken, which is causing the tests to sleep for 30 seconds repeatedly.

Coverage reporting is happening in the makefile in these lines:

• https://github.com/m3db/m3/blob/master/Makefile#L360
• https://github.com/m3db/m3/blob/master/Makefile#L369
• https://github.com/m3db/m3/blob/master/Makefile#L378

This is the script that is doing the coverage reporting: https://github.com/m3db/ci-scripts/blob/53a70df5b14add46dbcbcbc28d91dbf6fd7cd683/codecov.sh

The temporary fix right now is to pause coverage reporting, so that our pipeline tests run faster whenever someone is trying to create a PR. We are using SKIP_CODECOV env variable to pause coverage reporting.