Add exclusion for helix .bck files

[David-Else]

This should fix https://github.com/gohugoio/hugo/issues/12677, see https://github.com/gohugoio/hugo/issues/12677#issuecomment-2308489571 by @jmooring

[CLAassistant]

All committers have signed the CLA.

[bep]

This should fix #12677

Can someone test this and confirm. I'm not a fan of "I think this should work" when it can be easily tested before we merge it.

[David-Else]

I just tested it by building my branch (first time at Go), and it works great, you merged before I had time to get back to you :)

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