feat(ci): Add post script stage to seed.yml configs

[chrismallinson]

Description

Linear ticket: https://linear.app/buildwithfern/issue/FER-6998/ci-add-postscript-cleanup-stage-to-config and https://linear.app/buildwithfern/issue/FER-7247/ci-make-jobs-cancellable
Closes FER-6998
Closes FER-7247
Virtual environments are being created and never removed during the seed generation. Also fixed during testing - give the ability to cancel any CI job (ones using always() need to be accompanied by && !cancelled()

Changes Made

• Attach a && !cancelled() to every always() in GitHub actions to prevent jobs from running when cancelled
• Turn back on python-sdk in nightly runs
• Change mkdir to mkdir -p in case folder already exists in scripts
• Added postScript cleanup in seed/python-sdk/seed.yml
• Added postScript to the seed workspace config
• Added cleanup method to ScriptRunner for running postScript stage of config

Testing

• Fixed python-sdk in nightly run: Verified in CI here: https://github.com/fern-api/fern/actions/runs/18633007985
• Making jobs cancellable: Verified in CI here: https://github.com/fern-api/fern/actions/runs/18632889160
• Rest of testing between docker and local runs of fixtures handled via command line. Verified post script stage runs after docker scripts unless --skipScripts is added. Still need to verify local runs (don't have environment setup to run python locally at the moment)