ci: check out the Daimon provenance fixture the source build needs

[apresmoi]

The publish workflow's Verify Git-free offline linux/amd64 source build step fails with:

ENOENT: no such file or directory, lstat '/home/runner/work/spawnfile/daimon/package.json'

scripts/source-provenance-bundle.integration.test.ts resolves Daimon's source from SPAWNFILE_TEST_DAIMON_SOURCE, falling back to a sibling ../daimon checkout that does not exist on a runner. The script already supports the env var — its own comment says "the way SPAWNFILE_TEST_MOLTNET_SOURCE already does for the Moltnet variant" — but the workflow never sets it, while the Moltnet verification immediately below does check out its fixture.

This mirrors the Moltnet steps exactly: check out noopolis/daimon into daimon-fixture, point the env var at it, remove it afterwards with if: always().

Found while publishing v0.1.17, which this blocks.

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