fix: Force JarCache to be on on JarCache tests

[blorente]

Checklist

• I have filed an issue about this change and discussed potential changes with the maintainers.
• I have received the approval from the maintainers to make this change.
• This is not a stylistic, refactoring, or cleanup change.

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Discussion thread for this change

Discovered while trying to set up CI for our internal mirror of the plugin.

Description of this change

The combination of running these tests from macOS in bazel doesn't seem to work for these tests, because the BlazeJavaUserSettings default to useJarCache=false when we're not running with Blaze.

To solve it, we:

• Force BlazeJavaUserSettings.useJarCache to true, because that's what we're testing.
• Make the tests believe that they are running under Bazel, because JarCaches are only allowed in Mac if they are under Bazel (and not Blaze).