Fix Bundler config scope for custom Gemfiles

What does this MR do?

This MR ensures that all Bundler commands use the same application context when GITLAB_BUNDLE_GEMFILE points to a nested Gemfile.

The root .bundle/config entry is retained because the installed-package runtime helper uses it to locate the custom Gemfile. An absolute BUNDLE_GEMFILE is also passed to subsequent Bundler commands so that force_ruby_platform, build.*, frozen, and without settings are written to and read from the custom application root consistently.

This fixes native extension build options being silently ignored for custom Gemfiles, including:

• io-event --disable-epoll_pwait2
• ruby-magic --with-magic-flags=--disable-zstdlib
• force_ruby_platform

The default root Gemfile build path is unchanged.

Changelog: fixed

Related issues

Closes #10076 (closed)

Related to:

• !6207 (merged)
• !9563 (closed)

Test plan

The behavior was reproduced and verified with:

• Bundler 4.0.12
• Ruby 3.3.12
• AlmaLinux 9 builder image revision 5.65.0
• GITLAB_BUNDLE_GEMFILE=jh/Gemfile

Before this change:

ruby extconf.rb
epoll_pwait2@GLIBC_2.35
magic.so -> libzstd.so.1

After this change:

ruby extconf.rb --disable-epoll_pwait2
ruby extconf.rb --with-magic-flags=--disable-zstdlib

The resulting native extensions no longer reference epoll_pwait2@GLIBC_2.35 or depend on libzstd.so.1.