fix: include the toolchain interpreter in rb_gem_push runfiles

[titusfortner]

Problem

• rb_gem_push only adds the toolchain's gem script and the .gem file to its runfiles, so the generated wrapper's rlocation of the interpreter comes up empty and the final exec ruby|jruby gem push ... falls back to whatever is on PATH. With MRI this is masked by a system Ruby on most machines; with JRuby it fails outright:

dirname: missing operand
bazel-out/k8-opt/bin/rb/selenium-webdriver-release-nightly.sh: line 104: exec: jruby: not found

• The rule also skips the toolchain env (TruffleRuby's OPENSSL_PREFIX and LANG, JRuby's Windows JAVA_OPTS) and only applies the target's env through RunEnvironmentInfo, so the documented $(location) expansion never runs and the generated script only works under bazel run.

Seen in the SeleniumHQ/selenium nightly release: https://github.com/SeleniumHQ/selenium/actions/runs/35165684730

Solution

• Mirror rb_binary: put toolchain.files in runfiles, pass java_bin so JAVA_HOME resolves from runfiles under JRuby, merge toolchain.env, and bake the env into the generated script.
• Add bazel run :gem-release -- --help to the examples/gem CI matrix. It fails on every JRuby entry without this change.

Verified in Selenium with the runfiles part applied as a patch: https://github.com/SeleniumHQ/selenium/actions/runs/35171062663/job/105043434920

[p0deje]

@titusfortner Can you check what's wrong with CI?

[titusfortner]

The Windows failures were two pre-existing rb_gem_push bugs that the new bazel run :gem-release CI step surfaced.

• the batch wrapper unconditionally calls rlocation with {runfiles_helper}, which rb_gem_push never supplied, so every Windows job exited with "Expected two arguments for rlocation function" before gem ran. Fixed by wiring the runfiles helper into rb_gem_push the same way rb_binary does.
• JRuby's jgem.cmd is a plain batch file, so jruby.exe jgem.cmd fails to parse. MRI only works because Ruby's gem.cmd is a polyglot that is valid Ruby, so the fix bypasses the interpreter for JRuby .cmd stubs only.