Include PHPCS path in `$PATH`. Install WP standards for PHPCS.

Before:

```bash
/srv/www/phpcs/scripts/phpcs --standard=CodeSniffer/Standards/WordPress/WordPress/ruleset.xml ./htdocs/wp-load.php
```

After:
```bash
phpcs --standard=WordPress ./htdocs/wp-load.php
```
Sorry, it seems it has to start with an explicit `.`. Here's what I have:

```
phpcs --config-show installed_paths
Array
(
    [installed_paths] => ./CodeSniffer/Standards/WordPress/
)
```