fix(tower-uv): use copy link mode when no cache dir is set

[bradhe]

When a Python app is set up, uv occasionally prints this warning into the setup logs:

warning: Failed to hardlink files; falling back to full copy. This may lead to degraded performance.
         If the cache and target directories are on different filesystems, hardlinking may not be supported.
         If this is intentional, set \`export UV_LINK_MODE=copy\` or use \`--link-mode=copy\` to suppress this warning.

Root cause

When tower_uv::Uv is constructed without an explicit cache dir, uv uses its default cache (~/.cache/uv). In sandboxed execution the default cache and the target virtualenv frequently live on different filesystems, so uv can't hardlink across them and falls back to copying, emitting the warning. It looks intermittent because it only surfaces on runs whose app actually has dependencies to materialize into the venv.

The copy fallback was already happening; the warning is just noise.

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