Keep CI green paths independent: silence pylint false positives, don't fail-fast on 3.15-dev

[wesleybl]

Two independent CI-hardening fixes, no runtime changes:

1. Silence arguments-differ false positives (W0221) in
   src/relstorage/adapters/interfaces.py and
   src/relstorage/cache/interfaces.py. zope.interface method
   declarations intentionally omit self by convention; newer
   pylint/astroid (4.x, as installed on the Python 3.14 lint job)
   compares them against InterfaceBase/InterfaceClass dunders
   (__call__, __str__, __contains__, __iter__) and reports a
   mismatch. The suppression follows the pattern these files already
   use for other interface-style checks
   (no-self-argument, signature-differs, ...).

2. Set fail-fast: false on the test matrix so the known
   Python 3.15-dev segfault (a perfmetrics C-accelerator issue,
   under investigation upstream — unrelated to this change) does not
   cancel the remaining matrix jobs. The 3.15-dev job itself stays
   red as a canary until the upstream issue is resolved.

[wesleybl]

@jamadden The Python 3.15 job is failing for reasons unrelated to relstorage. We have therefore prevented this failure from cancelling the other jobs.

Could you please start the GitHub Actions?

[wesleybl]

See: zodb/perfmetrics#44