Fix wasm:install_jars on JRuby 10.1.2.0

[ksss]

The jruby workflow has been failing on master since ruby/setup-ruby was bumped to v1.327.0 (#3182), which moved the job from JRuby 10.1.1.0 to 10.1.2.0:

rake aborted!
Errno::ENOENT: No such file or directory - /home/runner/work/rbs/gems/rbs-4.3.0.dev-java/deps.lst
/home/runner/work/rbs/rbs/Rakefile:1001:in 'block in <main>'

Cause

JRuby 10.1.2.0 bundles jar-dependencies 0.6.1 (10.1.1.0 had 0.5.7). Jars::Installer writes a temporary deps.lst into MavenExec#basedir, which is spec.gem_dir when given a Gem::Specification. For Gem::Specification.load("rbs.gemspec") in a checkout, that is a nonexistent <checkout>/../gems/rbs-*-java directory.

• 0.5.7 passed the path to Maven as outputFile, and Maven silently created the directory (leaving a stray ../gems/rbs-*-java/lib/rbs_jars.rb behind).
• 0.6.1 resolves with Mima and writes the file with a plain File.open, so it fails with ENOENT.

I reproduced this locally with both JRuby distributions: 10.1.2.0 fails with the same error, and it even passes if a 10.1.1.0 run has already created the stray directory.

Fix

Pass the gemspec path to Jars::Installer, so the basedir is the checkout. Instead of patching spec.platform afterwards, RBS_PLATFORM=java is set while the gemspec is evaluated, so the gemspec stamps the java platform itself (Jars::Installer skips non-java gems). The env var is restored afterwards.

With this change, jruby -S rake wasm:install_jars succeeds locally on both JRuby 10.1.1.0 and 10.1.2.0 (Temurin 21), and nothing is written outside the checkout.

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