Runner: bug fix - disable cache when running Diff report

This fixes the following fatal error which would occur if the `Diff` report is used in combination with caching:
```
Fatal error: Uncaught Error: Call to a member function getFixableCount() on null in path/to/PHP_CodeSniffer/src/Fixer.php:144
Stack trace:
#0 path/to/PHP_CodeSniffer/src/Reports/Diff.php(73): PHP_CodeSniffer\Fixer->fixFile()
#1 path/to/PHP_CodeSniffer/src/Reporter.php(285): PHP_CodeSniffer\Reports\Diff->generateFileReport(Array, Object(PHP_CodeSniffer\Files\LocalFile), true, 150)
#2 path/to/PHP_CodeSniffer/src/Runner.php(659): PHP_CodeSniffer\Reporter->cacheFileReport(Object(PHP_CodeSniffer\Files\LocalFile))
#3 path/to/PHP_CodeSniffer/src/Runner.php(400): PHP_CodeSniffer\Runner->processFile(Object(PHP_CodeSniffer\Files\LocalFile))
#4 path/to/PHP_CodeSniffer/src/Runner.php(119): PHP_CodeSniffer\Runner->run()
#5 path/to/PHP_CodeSniffer/bin/phpcs(30): PHP_CodeSniffer\Runner->runPHPCS()
#6 {main}
  thrown in path/to/PHP_CodeSniffer/src/Fixer.php on line 144
```

Author: jrfnl

src/Runner.php

@@ -117,6 +117,13 @@ public function runPHPCS()
                 $this->config->cache = false;
             }
 
+            // Disable caching if the Diff report has been requested as it will run
+            // the fixer and the fixer doesn't use/support the cache functionality.
+            $requestedReports = array_change_key_case($this->config->reports, CASE_LOWER);
+            if (array_key_exists('diff', $requestedReports) === true) {
+                $this->config->cache = false;
+            }
+
             $numErrors = $this->run();
 
             // Print all the reports for this run.