Disable coverage builder by default for PRs
Disable coverage builder by default for PRs

Only run the coverage builder for a PR when requested to do.  The
builder can be requested by adding the `Requires-builders:` keyword
and a list of builders to the git commit message.  For example, the
following line will request all current builders:

Requires-builders: arch,style,amazon2,centos7,centos8,debian10,
    fedora32,ubuntu18,ubuntu20,builtin,freebsd12,freebsd13,coverage

The coverage build is still run on all merges to master.

Signed-off-by: Brian Behlendorf <32d50ad1e384dcb9a5bf7e40fa470df5dabaf348@llnl.gov>

zfsonlinux/zfs-buildbot,zfsonlinux/zfs-buildbot