Fix buildSrc:test failing on JDKs before JDK 25 with Non-English locale

[quaff]

See #35777 (comment)

[ChildrenGreens]

I am seeing the same MultiReleaseJarPluginTests.validateJar() failure on a non-English Windows environment.

Environment:

• OS: Windows 11 (zh_CN)
• JDK: 25 (C:\Program Files\Java\jdk-25)
• Console encoding: GBK

The error message printed by the validateMultiReleaseJar task is localized and garbled, for example:

Task :validateMultiReleaseJar FAILED
��Ŀ META-INF/versions/17/Main.class ����������������ڰ汾��ͬ�� api

Because of this, the assertion

assertThatThrownBy(...).hasMessageContaining(
    "entry: META-INF/versions/17/Main.class, contains a class with different api from earlier version"
);

does not match, and the test fails.

This PR makes sense to me since it forces user.language=en for the TestKit build and should fix the issue for non-English locales as well.

[bclozel]

Thanks @quaff !