Bump github.com/spf13/viper from 1.8.1 to 1.9.0 (ko-build#442)

Bumps [github.com/spf13/viper](https://github.com/spf13/viper) from 1.8.1 to 1.9.0.
- [Release notes](https://github.com/spf13/viper/releases)
- [Commits](spf13/viper@v1.8.1...v1.9.0)

---
updated-dependencies:
- dependency-name: github.com/spf13/viper
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

go.mod

@@ -17,7 +17,7 @@ require (
 	github.com/opencontainers/image-spec v1.0.2-0.20210730191737-8e42a01fb1b7
 	github.com/spf13/cobra v1.2.1
 	github.com/spf13/pflag v1.0.5
-	github.com/spf13/viper v1.8.1
+	github.com/spf13/viper v1.9.0
 	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
 	golang.org/x/time v0.0.0-20210723032227-1f47c861a9ac // indirect
 	golang.org/x/tools v0.1.6