build/pkgs/giac: demote to optional

This is now strictly required only by the (optional) sagemath_giac
package. There is a feature test guarding the doctests in the old
pexpect interface under sage.interfaces.giac, and nothing else in
sagelib imports that pexpect interface, so if you do not use it
explicitly, giac is optional.

build/pkgs/giac/type

@@ -1 +1 @@
-standard
+optional