dnf sbom: handle arch absence

[Habbie]

Short description

fixes the breakage introduced in #14691

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[Habbie]

non-release dnsdist (because auth builds don't use the extra json args) test build at https://github.com/Habbie/pdns/actions/runs/10963274205

[coveralls]

Pull Request Test Coverage Report for Build 10963070747

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 45 unchanged lines in 12 files lost coverage.
• Overall coverage increased (+3.4%) to 64.654%

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Files with Coverage Reduction	New Missed Lines	%
pdns/dnsdistdist/dnsdist-crypto.cc	2	75.72%
pdns/sstuff.hh	2	56.83%
modules/lmdbbackend/lmdbbackend.cc	2	73.46%
pdns/dnsdistdist/dnsdist.cc	2	68.36%
pdns/recursordist/test-syncres_cc2.cc	3	88.91%
pdns/stubresolver.cc	3	77.58%
pdns/recursordist/aggressive_nsec.cc	4	66.1%
pdns/packethandler.cc	4	72.47%
pdns/recursordist/syncres.cc	5	79.36%
pdns/signingpipe.cc	5	84.99%

Totals
Change from base Build 10961070589:	3.4%
Covered Lines:	124759
Relevant Lines:	162212

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💛 - Coveralls

[Habbie]

fake dnsdist release build https://github.com/Habbie/pdns/actions/runs/10964295787