fix: Fixes "X is a directory" error on output

What does this MR do?

Fixes "X is a directory" error when attempting to read custom ruleset configuration

Reproduction

Before

❯ docker run -it --rm --volume "$PWD":/tmp/app --env CI_PROJECT_DIR=/tmp/app --env SECURE_LOG_LEVEL=warn --env GITLAB_FEATURES=sast_custom_rulesets -w /tmp/app registry.gitlab.com/gitlab-org/security-products/analyzers/bundler-audit:2
[ERRO] [bundler-audit] [2021-11-09T17:14:07Z] ▶ read /tmp/app: is a directory

After

❯ docker run -it --rm --volume "$PWD":/tmp/app --env CI_PROJECT_DIR=/tmp/app --env SECURE_LOG_LEVEL=warn --env GITLAB_FEATURES=sast_custom_rulesets -w /tmp/app bundler-audit:master

What are the relevant issue numbers?

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer