buildflags: error on duplicate attest field

[jedevc]

🐛 Bug report from @tianon:

--attest type=sbom,generator=... did work, but I had --sbom=true later in my CLI which then overwrote the whole --attest type=sbom with one that had no arguments

We now explicitly error if a single field is repeated (I seem to remember we did this at some point previously, but looks like it was removed).