fix: e2e(pycobertura): failing after test-env guard; unblock PR #1210 (fixes #1211)

[krystophny]

Summary

• Add CI override FORTCOV_ALLOW_AUTOTEST to bypass test-env guard when safe.
• Set job env in e2e_pycobertura to keep auto paths available in CI.

Verification

• Pytest: pytest -q --maxfail=1 --disable-warnings
  Output: 3 passed in 0.05s
• fpm tests: /usr/bin/timeout 120s fpm test
  Excerpts:
  • OK: cli flags rejection minimal
  • OK: test_env_guard override bypasses test detection when enabled
• Files:
  • src/utils/runtime/test_env_guard.f90
  • test/test_test_env_guard_override.f90
  • .github/workflows/e2e_pycobertura.yml
  • fpm.toml

Rationale

• E2E job failed after adding test-environment guard which skips auto-test/bridge.
• Introducing a CI-only opt-in env keeps guards intact for tests while unblocking E2E.