Fix Checkstyle to rely on the absolute path

It turns out that relative path in the property
`<property name="file" value="src/checkstyle/checkstyle-suppressions.xml"/>`
in some cases is resolved against a Gradle deamon dir as a root for configs

* Fix Checkstyle plugin config to use an absolute path in the project for
the `configDirectory`.
* Since `configDirectory` is exposed as a Checkstyle config variable,
we can use its `${config_loc}` placeholder to rely on the absolute path
in the project

build.gradle

@@ -176,7 +176,7 @@ subprojects { subproject ->
 	}
 
 	checkstyle {
-		configFile = file("${rootDir}/src/checkstyle/checkstyle.xml")
+		configDirectory.set(file("src/checkstyle"))
 		toolVersion = '8.24'
 	}
 

src/checkstyle/checkstyle.xml

@@ -5,12 +5,12 @@
 <module name="Checker">
 
 	<module name="SuppressionFilter">
-		<property name="file" value="src/checkstyle/checkstyle-suppressions.xml"/>
+		<property name="file" value="${config_loc}/checkstyle-suppressions.xml"/>
 	</module>
 
 	<!-- Root Checks -->
 	<module name="RegexpHeader">
-		<property name="headerFile" value="src/checkstyle/checkstyle-header.txt"/>
+		<property name="headerFile" value="${config_loc}/checkstyle-header.txt"/>
 		<property name="fileExtensions" value="java"/>
 	</module>
 	<module name="NewlineAtEndOfFile">