buildbox-run-userchroot may use PATH to find `argv[0]` of command

Context

The Command message of the Remote Execution API specifies the following about argv[0]:

The first argument must be the path to the executable, which must be either a relative path, 
in which case it is evaluated with respect to the input root, or an absolute path.

buildbox-run-userchroot doesn't respect this, as the wrapper script it creates to setup the environment/run the command uses exec. We should fix this so that a change in the $PATH variable doesn't influence what argv[0] points to.

Acceptance Criteria

buildbox-run-userchroot doesn't rely on $PATH to find a relative path executable, and instead finds it with respect to the input_root instead.