Clarify SBOM output debug message

[Dentrax]

apko does emit SBOM by default (since --sbom generate SBOMs (default true) is true). If you pass --debug flag to debug, the last line of build output is:

Nov 21 11:15:46.459 [DEBUG] [arch:x86_64] Not generating SBOMs (WantSBOM = false)

I expect not to see this warning, since (--debug is true). Better omit this. Or we can rephrase this:

Generating SBOMs (WantSBOM = true)

cc @developer-guy