fix(ci): allow pre-releases when pinning the nightly in migration validation
Migration Test: pip/venv (stable -> nightly) failed deterministically on
nightly run 27260425158 (twice, including a rerun):
hint: langflow-base was requested with a pre-release marker (e.g.,
langflow-base==1.11.0.dev1), but pre-releases weren't enabled
(try: --prerelease=allow)
This is the first nightly publishing as a canonical .devN pre-release
of the langflow distribution (nightly -> stable bundle cutover). The
'latest' branch of the upgrade step already passes --prerelease=allow,
but the pinned-version branches do not. uv implicitly allows the
pre-release for the directly requested ==X.Y.Z.devN pin, yet langflow's
metadata pins langflow-base==X.Y.Z.devN transitively, and uv rejects
transitive pre-releases unless they are enabled - so the install fails
after the stable uninstall, sinking the migration test.
Add --prerelease=allow to both pinned-version install lines.
75e01157fe