Re-initialise field on setClasspath

Without that change, it became illegal to do something like
`setClasspath(files(someFile, getClasspath())` which can be used to
prepend values to the classpath.

Fixes #8748

subprojects/core/src/main/java/org/gradle/process/internal/JavaExecHandleBuilder.java

@@ -207,7 +207,9 @@ public List<CommandLineArgumentProvider> getArgumentProviders() {
     }
 
     public JavaExecHandleBuilder setClasspath(FileCollection classpath) {
-        doGetClasspath().setFrom(classpath);
+        ConfigurableFileCollection newClasspath = fileCollectionFactory.configurableFiles("classpath");
+        newClasspath.setFrom(classpath);
+        this.classpath = newClasspath;
         return this;
     }
 

subprojects/core/src/test/groovy/org/gradle/process/internal/JavaExecHandleBuilderTest.groovy

@@ -20,6 +20,7 @@ import org.gradle.api.internal.file.FileCollectionFactory
 import org.gradle.api.internal.file.TestFiles
 import org.gradle.initialization.DefaultBuildCancellationToken
 import org.gradle.internal.jvm.Jvm
+import spock.lang.Issue
 import spock.lang.Specification
 import spock.lang.Unroll
 
@@ -104,6 +105,22 @@ class JavaExecHandleBuilderTest extends Specification {
         builder.classpath.contains(jar2)
     }
 
+    @Issue("gradle/gradle#8748")
+    def "can prepend to classpath"() {
+        given:
+        File jar1 = new File("file1.jar").canonicalFile
+        File jar2 = new File("file2.jar").canonicalFile
+
+        builder.classpath(jar1)
+
+        when:
+        builder.setClasspath(fileCollectionFactory.resolving(jar2, builder.getClasspath()))
+
+        then:
+        builder.commandLine.contains("$jar2$File.pathSeparator$jar1".toString())
+
+    }
+
     def "detects null entries early"() {
         when: builder.args(1, null)
         then: thrown(IllegalArgumentException)