Add –free-up-disk-before-build option to getdeps generate-github-actions

Summary: Add a new –free-up-disk-before-build CLI flag to the generate-github-actions command. When set, a step is emitted immediately before the final project build that removes large system tools (dotnet, powershell, ghcup) to free disk space. This is a no-op on Windows.

Use this flag for the rebalancer Linux CI workflow, restoring the disk cleanup step that was previously hand-edited into the workflow but not tracked in the generator script.

Reviewed By: sahilsd

Differential Revision: D101254559

fbshipit-source-id: bfd281eb0f746082d7ca68f8e15447511fda46a8