Fix tool output mangled with spawned cmd string

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.

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 actions#649.

packages/exec/src/toolrunner.ts

@@ -418,8 +418,8 @@ export class ToolRunner extends events.EventEmitter {
       }
 
       const optionsNonNull = this._cloneExecOptions(this.options)
-      if (!optionsNonNull.silent && optionsNonNull.outStream) {
-        optionsNonNull.outStream.write(
+      if (!optionsNonNull.silent && optionsNonNull.errStream) {
+        optionsNonNull.errStream.write(
           this._getCommandString(optionsNonNull) + os.EOL
         )
       }