Problem

Custom build packaging tasks can fail on fresh hosts while applying version expansions. The custom version override is invoked before the Python virtual environment is initialized, so its Python dependency imports fail with exit code 1.

Root Cause

The shared version-expansion flow applies the custom build version override before venv setup. The override path depends on the venv, but the ordering assumes that dependency initialization has already occurred.

Scope of Work

• Move custom version override application until after venv initialization.
• Keep the shared version-expansion function independent of venv initialization.
• Reapply the custom version after fetching shared expansions on package-test paths so custom artifact versioning is preserved.
• Add coverage that verifies the command ordering.

Acceptance Criteria

• Custom build version expansion completes on fresh hosts without dependency-import failures.
• Custom artifact/package version overrides remain effective.
• The command-ordering test prevents regressions.

References

• BF-45943
• PR #61753
• Evergreen patch #6879 (running; custom_build_deliver excluded)