Fix bundle.bat re-execution

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

Failing CI in https://github.com/rubygems/compact_index/ under mswin, because Bundler was trying to run a .bat script using ruby:

$ C:\Windows\system32\cmd.exe /D /S /C "D:\ruby-mswin\bin\bundle.bat install --jobs 4"
Bundler 2.6.0.dev is running, but your lockfile was generated with 2.4.12. Installing Bundler 2.4.12 and restarting using that version.
Fetching gem metadata from https://rubygems.org/.
Fetching bundler 2.4.12
Installing bundler 2.4.12
D:/ruby-mswin/bin/bundle: --> D:/ruby-mswin/bin/bundle
'$(' is not allowed as a global variable nameunexpected '(', expecting end-of-input
  2  {
> 3    bindir=$(dirname "$0")
> 4    exec "$bindir/ruby" "-x" "$0" "$@"
  5  }
D:/ruby-mswin/bin/bundle:3: syntax errors found (SyntaxError)
  1 | #!
  2 | {
> 3 |   bindir=$(dirname "$0")
    |          ^~ '$(' is not allowed as a global variable name
    |           ^ expected a `=>` between the hash key and value
> 4 |   exec "$bindir/ruby" "-x" "$0" "$@" ...
    | ^ expected a `}` to close the hash literal
    |   ^~~~ unexpected local variable or method, expecting end-of-input
> 5 | }
    | ^ unexpected '}', ignoring it
  6 | #!/usr/bin/env ruby
  7 | #

What is your fix for the problem, implemented in this PR?

.bat scripts don't have ruby code, so we should not prepend ruby to the command the re-execute it.

This fixes https://github.com/rubygems/compact_index CI as proved by rubygems/compact_index#67.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
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[deivid-rodriguez]

I'll merge to get https://github.com/rubygems/compact_index/ CI back on track.