We currently use the absolute path of these binaries in the
helper. This has some advantages but given that we control the
inputs for PATH in general it seems unnecessary.
We are also slightly inconsistent about this in the codebase but
favor the non absolute path version. A quick count:
```
$ git grep '"chroot"'|wc -l
13
$ git grep '"/usr/sbin/chroot"'|grep -v test_|wc -l
8
```
for `mount` and `umount` it seems this is the only place that uses
the absolute path.
It's not an important change but it has the nice property that it
allows us to use e.g. `testutil.mock_command()` in our tests and
it would be nice to be consistent.
a3e32f3823