make(1): improve const-correctness in archive handling

The return value of Var_Value must not be modified.  Ideally it would be
declared as const char *, but that still takes a while, especially since
much of the make code is not yet covered by the unit tests.

The variable cp had to be changed to const char * as well, and while here
was split up into one variable per actual use case.

(rillig)