run-as: reduce the scope of changed egid.

There's no bug here that I can see, but having a larger than necessary
scope just seems like a bad idea in case anyone adds anything later.

Also switch to modern struct initialization syntax.

Bug: http://b/308990322
Test: treehugger
Change-Id: If2576f902de62e8c24d4a643c224df8ea6cff5d5