Fix tool output mangled with spawned cmd string

[OJFord]

Presently if the exec'd tool is expected to produce an output (i.e. to write to stdout) that can be consumed in a Unix pipeline, assigned to a variable, or similar; the silent option must be set in order to avoid the output being mangled by @actions/exec to include the full command string that it spawns (and then re-writing out from the provided stdout listener).

This commit writes the spawned command string to stderr instead of stdout, as would be expected for logging/debug information as opposed to consumable output data.

Closes #649.

[OJFord]

@bdehamer I've read contributing, have I missed a CLA or something? Would appreciate a review when someone has a chance; still here and happy to update as necessary.