gradle: Improve toolchains test

Previously the test relied on the output of the javaToolchains task and
therefore on the way that toolchains are passed to Gradle. This was
brittle in case we want to change the way of how we supply toolchains to
Gradle (see e.g. #366929).

After this change, instead of inspecting the output of javaToolchains,
the toolchains test now tries to assemble a project using a toolchain
that was passed via the gradle.override.

pkgs/development/tools/build-managers/gradle/default.nix

···
 {
-  jdk11,
   jdk17,
   jdk21,
   jdk23,
···
             unwrapped = gradle;
             tests = {
               toolchains =
+                let
+                  javaVersion = lib.substring 0 2 (lib.getVersion jdk23);
+                in
                 runCommand "detects-toolchains-from-nix-env"
                   {
                     # Use JDKs that are not the default for any of the gradle versions
                     nativeBuildInputs = [
                       (gradle.override {
                         javaToolchains = [
-                          jdk11
                           jdk23
                         ];
                       })
                     ];
-                    src = ./tests/java-application;
+                    src = ./tests/toolchains;
                   }
                   ''
                     cp -a $src/* .
+                    substituteInPlace ./build.gradle --replace-fail '@JAVA_VERSION@' '${javaVersion}'
                     env GRADLE_USER_HOME=$TMPDIR/gradle org.gradle.native.dir=$TMPDIR/native \
-                      gradle javaToolchains --no-daemon --quiet --console plain > $out
-                    cat $out | grep "Language Version:   11"
-                    cat $out | grep "Detected by:        environment variable 'JAVA_TOOLCHAIN_NIX_0'"
-                    cat $out | grep "Language Version:   23"
-                    cat $out | grep "Detected by:        environment variable 'JAVA_TOOLCHAIN_NIX_1'"
+                     gradle run --no-daemon --quiet --console plain > $out
+                    grep -q "JAVA_VERSION: ${javaVersion}" $out || exit 1
                   '';
             } // gradle.tests;
           }

pkgs/development/tools/build-managers/gradle/tests/toolchains/build.gradle

···
+plugins {
+    id('application')
+}
+
+java {
+    toolchain.languageVersion.set(JavaLanguageVersion.of(@JAVA_VERSION@))
+}
+
+application {
+    mainClass = 'Main'
+}

pkgs/development/tools/build-managers/gradle/tests/toolchains/src/main/java/Main.java

···
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("JAVA_VERSION: " + System.getProperty("java.version"));
+    }
+}

pkgs/top-level/all-packages.nix

···
   gnumake = callPackage ../development/tools/build-managers/gnumake { };
   gradle-packages = import ../development/tools/build-managers/gradle {
     inherit
-      jdk11
       jdk17
       jdk21
       jdk23