Tell rustc to always use color output

By default, rustc will disable color output when it detects that its
stdin isn’t connected to a terminal. However, since we will print the
output to the user’s terminal, it makes sense to override this.

Test: Manually by triggering a compilation error. I ran
  * m pdl
  * atest --host --no-bazel-mode pdl_inline_tests
  * atest pdl_inline_tests
  and got useful color output in my terminal in each case.
Change-Id: I8b593d2eb59d13db17a3456e3fb053556fc16f07