rust: Remove unnecessary escaping in grep call

Modules with special characters wouldn't build correctly because $out
was being double escaped -- once in the build command and once by Soong
itself.

Remove the escape in the build command and just let Soong escape as
necessary.

Bug: 320578837
Test: Module with @ in name builds
Change-Id: Ic2554b9c07f37783287e1ebdeb06ef8b7b66a296