[goreleaser] specify branch name for PR (#137)

This property defaults to the repo's default branch which is causing
goreleaser to attempt to merge directly to `main`. Setting this property
to another branch name should hopefully allow it to create a PR and
successfully update the tap.

.goreleaser.yml

@@ -42,6 +42,7 @@ brews:
  - repository:
  owner: mercari
  name: hcledit
+ branch: update-brew-formula
  pull_request:
  enabled: true
  base: