ci: shared gh-ci scripts: reject non-Linux matrix cells, CRLF METADATA, matrix.py parsing #385

Follow-up from the review of #378 (latent for dokimi, which builds on Linux only):

• intel-ci.yml: runs-on ignores matrix.platform. Make matrix.py reject non-Linux cells, rather than mapping them, until a non-Linux runner is wanted.
• build.py add_requirements: handle CRLF line endings in METADATA.
• matrix.py: a skip line with a trailing comment; >3.10 specifiers; FreeBSD classifier.

These live in the shared gh-ci scripts, so fix them at the source and re-vendor.