Polish Gradle expression references

Expand all expression `$` references to the full `${...}` form.

spring-boot-tests/spring-boot-integration-tests/spring-boot-launch-script-tests/app/build.gradle

@@ -6,7 +6,7 @@ plugins {
 apply plugin: 'io.spring.dependency-management'
 
 repositories {
-	maven { url "file:$rootDir/../int-test-maven-repository"}
+	maven { url "file:${rootDir}/../int-test-maven-repository"}
 	mavenCentral()
 	maven { url 'https://repo.spring.io/snapshot' }
 	maven { url 'https://repo.spring.io/milestone' }

spring-boot-tests/spring-boot-integration-tests/spring-boot-launch-script-tests/app/settings.gradle

@@ -1,6 +1,6 @@
 pluginManagement {
 	repositories {
-		maven { url "file:$rootDir/../int-test-maven-repository"}
+		maven { url "file:${rootDir}/../int-test-maven-repository"}
 		mavenCentral()
 		maven { url 'https://repo.spring.io/snapshot' }
 		maven { url 'https://repo.spring.io/milestone' }

spring-boot-tests/spring-boot-integration-tests/spring-boot-launch-script-tests/build.gradle

@@ -22,12 +22,12 @@ dependencies {
 
 task syncMavenRepository(type: Sync) {
 	from configurations.app
-	into "$buildDir/int-test-maven-repository"
+	into "${buildDir}/int-test-maven-repository"
 }
 
 task syncAppSource(type: Sync) {
 	from 'app'
-	into "$buildDir/app"
+	into "${buildDir}/app"
 	filter { line ->
 		line.replace("id 'org.springframework.boot'", "id 'org.springframework.boot' version '${project.version}'")
 	}
@@ -35,7 +35,7 @@ task syncAppSource(type: Sync) {
 
 task buildApp(type: GradleBuild) {
 	dependsOn syncAppSource, syncMavenRepository
-	dir = "$buildDir/app"
+	dir = "${buildDir}/app"
 	startParameter.buildCacheEnabled = false
 	tasks  = ['build']
 }

spring-boot-tests/spring-boot-integration-tests/spring-boot-server-tests/build.gradle

@@ -35,7 +35,7 @@ dependencies {
 }
 
 task prepareMavenBinaries(type: org.springframework.boot.build.mavenplugin.PrepareMavenBinaries) {
-	outputDir = file("$buildDir/maven-binaries")
+	outputDir = file("${buildDir}/maven-binaries")
 	versions '3.6.2'
 }