add SYFT_IMAGE_TAG env var

[aptalca]

Allow defining of syft image tag via env var
Print syft image tag used in logs
Default syft image tag to latest

Needs linuxserver/docker-jenkins-builder#344 merged to master first

[LinuxServer-CI]

I am a bot, here is the pushed image/manifest for this PR:

ghcr.io/linuxserver/lspipepr-ci:a80a2536-pkg-a80a2536-dev-f74940f901addacae708700109ddffc058b296cb-pr-63

[thespad]

If we're defaulting to latest in linuxserver/docker-jenkins-builder#344 we should probably default to latest here too.

That said the causes of the 1.27.0 issues seem to be different between the two containers, so it might be better for now to leave this pinned at 1.26.1 - I'm not convinced it's even possible to work around the changes they've made.

Oh, also as it stands this will never fall back because the Jenkins builder defaults to latest if nothing is supplied to it so will always pass something to the CI container.

[aptalca]

Then we'll have to merge the jenkins builder PR first and make sure the jenkins var is set to the same v1.26.1 value so the ci container can then build and push.