fix(jinja): exhaustively handle CallArg variants in used-variable collection

[wolfv]

Summary

While fixing the minijinja 2.20 build break (PR #2427, Expr::Compare), I audited the rest of the AST walker in rattler_build_jinja/src/ast_variables.rs and found a pre-existing gap: collect_variables_from_call_arg matches only CallArg::Pos and uses _ => {} to swallow the other three variants. The same Pos-only filter is duplicated in the pin_subpackage, pin_compatible, and match special-function arms.

Result: templates using kwarg or splat call forms lose variables for variant tracking. E.g.

${{ pin_subpackage(name=somevar) }}    # somevar dropped
${{ my_fn(**kwargs) }}                 # kwargs dropped

In conda-build idioms these are always positional, so existing recipes are unlikely to hit this — but user-written macros that pass kwargs or splats would silently lose variant detection.

This PR:

• Adds call_arg_expr — a small helper that exhaustively destructures every CallArg variant (Pos | Kwarg | PosSplat | KwargSplat) into its inner Expr. No wildcard arm, so a future minijinja release that adds a new variant will fail to compile here instead of silently losing variables — same property that just saved us for Expr::Compare.
• Routes collect_variables_from_call_arg and the three special-function arms through the helper.
• Adds three regression tests: chained comparison (Expr::Compare), pin_subpackage kwarg form, and a generic call covering all four CallArg variants.

This is stacked on top of #2427 so the new chained-comparison test compiles. Once #2427 merges to main, GitHub will auto-rebase this diff down to just the audit changes.

Test plan

• cargo test -p rattler_build_jinja — 48/48 pass (up from 45)
• cargo check --workspace clean
• CI green

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