Use default config so ninja is in PATH

[bbondy]

config.defaultOptions contains an env which doesn't require ninja to be in your path. It already also includes both of these properties:

{
   stdio: 'inherit',
   shell: true
}

Without this PR, you'd need to have PATH="$PATH:/path/to/brave/vendor/depot_tools".

[garrettr]

Good catch! I should've tested #100 with an unmodified $PATH.