fix(report): send report to each slack channel

[tomofumi0003]

If this Pull Request is work in progress, Add a prefix of “[WIP]” in the title.

What did you implement:

fix vuls report not send to each server name's channel.
(when slack's channel is "${servername}" in configuration file)

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• set to slack's channel is "${servername}"
• vuls report -to-slack

Checklist:

You don't have to satisfy all of the following.

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES

Reference

[MaineK00n]

LGTM