Use os.Executable() to detect path to the agents own binary
Use os.Executable() to detect path to the agents own binary

We need the path to start the bootstrap as a child process. We've
previously detected it using os.Args[0], however there are some edge
cases where that doesn't work (eg #1439).

We think os.Executable() might be a more reliable way to detect the path.

buildkite/agent,buildkite/agent,buildkite/agent