ci: pass the test JDK vendor to the Gradle toolchain

The per-row post-processing in matrix.mjs read `java_vendor` from
`v.java_distribution` after that field had already been overwritten with
its string `value`, so `v.java_distribution.vendor` was always
`undefined`. As a result `jdkTestVendor` reached Gradle empty and the
test toolchain was never pinned to the matrix vendor, so tests could run
on any installed JDK of the matching version.

Destructure the axis object before overwriting the field, so the read no
longer depends on statement order.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: vlsi

.github/workflows/matrix.mjs

@@ -140,8 +140,9 @@ include.forEach(v => {
   // Gradle does not work in tr_TR locale, so pass locale to test only: https://github.com/gradle/gradle/issues/17361
   jvmArgs.push(`-Duser.country=${v.locale.country}`);
   jvmArgs.push(`-Duser.language=${v.locale.language}`);
-  v.java_distribution = v.java_distribution.value;
-  v.java_vendor = v.java_distribution.vendor;
+  const {value: javaDistribution, vendor: javaVendor} = v.java_distribution;
+  v.java_distribution = javaDistribution;
+  v.java_vendor = javaVendor;
   if (v.java_distribution === 'oracle') {
       v.oracle_java_website = v.java_version === eaJava ? 'jdk.java.net' : 'oracle.com';
   }