`make_packages_yaml` produces invalid `packages.yaml` file

[greenc-FNAL]

From https://dbweb9.fnal.gov:8443/ArtCI/app/storage/docs/2024/05/16/build_spack%23oN7g0Kt.log

178: spack.parser.SpecTokenizationError: unexpected tokens in the spec string
179: libc +iconv+rpc @-- %gcc@-- os=almalinux9
180:                 ^^^^    ^     ^          

[marcmengel]

I'm unable to reproduce this; everyplace I run this, I get:

libc:
- spec: "libc +iconv+rpc @2.34 os=almalinux9"

which parses fine. We've dropped the @gcc parts for newer Spack already, but the only way I can see getting the @-- is if something is very strange with the "rpm" command on the system.
Where did this run?!?