printf command not found

```sh
go tool cover -html=coverage.out -o coverage.html
/bin/sh: @printf: command not found
make[1]: *** [test-coverage] Error 1
make: *** [report] Error 2
```

This happens when running `make report`. I haven't dug into why it's happening but am guessing it has to do with my zsh shell.

Reproduce the failure with make report, then inspect the report and test-coverage targets and the command invoking go tool cover. Done means make report completes without the printf error and produces coverage.html from coverage.out.

Written by the indexing model from the issue text.