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

rust/config/global.go

@@ -50,6 +50,7 @@ var (
 		"-C force-unwind-tables=yes",
 		// Use v0 mangling to distinguish from C++ symbols
 		"-C symbol-mangling-version=v0",
+		"--color always",
 	}
 
 	deviceGlobalRustFlags = []string{