Don't hardcode user id in TarBackupReaderTest.

Test can fail if it is running on other user than system.

Fixing the HSUM failure b/394780895

Bug: 394780895
Flag: TEST_ONLY
Test: run abt tests on hsum target (see comment 13 in bug)
Change-Id: I16ad4c79b28f890b6047b7341648ec6a88e15f2c