ci: GHA: codecov: fail_ci_if_error=true

[blueyed]

This should make it obvious if missing coverage is due to the upload
having failed.

Using #6553 would be an
alternative, but this can be used already if that takes longer.

Ref: #6463 (comment)

[blueyed]

macos-py38:

 Codecov upload1m 15s
##[error]Node run failed with exit code 1
Run codecov/codecov-action@v1
/Users/runner/runners/2.164.0/work/_actions/codecov/codecov-action/v1/dist/index.js:292
      throw error;
      ^

Error: connect ETIMEDOUT 35.199.43.247:443
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1129:14) {
  errno: 'ETIMEDOUT',
  code: 'ETIMEDOUT',
  syscall: 'connect',
  address: '35.199.43.247',
  port: 443
}

[blueyed]

"py36" with the 2nd run:

Run codecov/codecov-action@v1
/home/runner/work/_actions/codecov/codecov-action/v1/dist/index.js:292
      throw error;
      ^

Error: connect ETIMEDOUT 35.199.43.247:443
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1129:14) {
  errno: 'ETIMEDOUT',
  code: 'ETIMEDOUT',
  syscall: 'connect',
  address: '35.199.43.247',
  port: 443
}

Closing in favor of #6553,