CI: run basictests on push to main only

[willend]

mcstas-basictest and mcxtrace-basictest used to run on both push and pull_request, so every push to a PR branch ran the full matrix twice.

push is now limited to main:

• PR branches are tested once, by the pull_request run (branch merged with main).
• main is still tested on merge. The Sunday cron and workflow_dispatch are unchanged.
• A branch without a PR gets no automatic run. Use "Run workflow" on it, or open a draft PR.

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