feat(flag): add error when there are no supported security checks

[DmitriyLewen]

Description

Trivy doesn't stop scan if --security-checks flag has only unsupported values.
We need to return error when parsing the flags.

Before:

➜ trivy -d image --security-checks conf alpine:3.14.3           
2022-08-16T10:41:50.312+0600    DEBUG   Severities: ["UNKNOWN" "LOW" "MEDIUM" "HIGH" "CRITICAL"]
2022-08-16T10:41:50.312+0600    WARN    unknown security check: conf
2022-08-16T10:41:50.315+0600    DEBUG   cache dir:  /home/dmitriy/.cache/trivy
2022-08-16T10:41:50.316+0600    DEBUG   Module dir: /home/dmitriy/.trivy/modules
2022-08-16T10:41:52.786+0600    DEBUG   Image ID: sha256:0a97eee8041e2b6c0e65abb2700b0705d0da5525ca69060b9e0bde8a3d17afdb
2022-08-16T10:41:52.786+0600    DEBUG   Diff IDs: [sha256:1a058d5342cc722ad5439cacae4b2b4eedde51d8fe8800fcf28444302355c16d]
2022-08-16T10:41:52.786+0600    DEBUG   Base Layers: []

After:

➜  ./trivy -d image --security-checks conf alpine:3.14.3 
2022-08-16T10:42:18.065+0600    DEBUG   Severities: ["UNKNOWN" "LOW" "MEDIUM" "HIGH" "CRITICAL"]
2022-08-16T10:42:18.065+0600    WARN    unknown security check: conf
2022-08-16T10:42:18.065+0600    FATAL   flag error:
    github.com/aquasecurity/trivy/pkg/commands.NewImageCommand.func2
        /home/dmitriy/work/aquasecurity/trivy/pkg/commands/app.go:267
  - scan flag error:
    github.com/aquasecurity/trivy/pkg/flag.(*Flags).ToOptions
        /home/dmitriy/work/aquasecurity/trivy/pkg/flag/options.go:366
  - --security-check flag doesn't contain supported values:
    github.com/aquasecurity/trivy/pkg/flag.(*ScanFlagGroup).ToOptions
        /home/dmitriy/work/aquasecurity/trivy/pkg/flag/scan_flags.go:80
exit status 1

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

security-checks can be empty when generating SBOM

[DmitriyLewen]

I didn't think about it. Thanks. Fixed.

[knqyf263]

Suggested change

	return nil, xerrors.New(fmt.Sprintf("unknown security check: %s", v))
	return nil, xerrors.Errorf("unknown security check: %s", v)

[DmitriyLewen]

Done