/bin/env was used to resolve a command name against PATH and pass
additional environment variables.
All of this can also be achieved using functionality already provided by
go's exec lib, which will then internally pass the appropriate arguments
to e.g. execve.
A non-zero exit code signifies a pipeline failure, but is not a fatal error in the agent.
Since exec reports this as exec.ExitError, this has to be handled explicitly.
This also fixes logs not being shown on build errors.
Since /tmp is writable by everybody, a user could precreate
/tmp/woodpecker with 777 permissions, allowing them to modify the
pipeline while it is being run, or preventing the pipeline from running.
And since os.MkdirAll error code wasn't checked, the same attacker
could have precreated the directory where the pipeline is executed to
mess with the run, allowing code execution under the UID of the
agent (who has access to the toke, to communicate with the server, which
mean a attacker could inject a fake agent, steal credentials, etc)
