Allow user to disable auto reshim with environment variable

# Intro
For this line of code [here](https://github.com/asdf-vm/asdf-ruby/blob/059cdee5b02c5a5569a5858e2a379f84dbdb95d7/rubygems-plugin/rubygems_plugin.rb#L5), allow users to skip it with an environment variable. For example:

```ruby
module ReshimInstaller
def install(options)
super
`asdf reshim ruby` unless ENV["ASDF_SKIP_RESHIM"]
end
end
```

# Context
I have modularized my code and broken it off into separate gems/engines that all live near the main app. The folder structure resembles the following:
```
app/
bin/
build/
...
engines/
engine1/
app/
...
Gemfile
...
gems/
gem1/
...
lib/
```

In order to better manage these gems/ engines which have some interdependencies, I have written a script that can loop through all gems and run bundler commands like `bundle update ...`, and `bundle install`.

When using a tool like `rbenv`, this script will run in a couple of minutes. When using `asdf`, it takes ~15 minutes, because `asdf` is calling `reshim` for every gem/ engine where it will run the bundler command. Allowing an easy way to disable the automatic `reshim` would be extremely helpful.

Start at rubygems-plugin/rubygems_plugin.rb, specifically the reshim call linked in the issue. Check how the plugin's install flow handles environment variables, then verify that ASDF_SKIP_RESHIM allows users to skip automatic reshim while preserving the current behavior when it is unset.

Written by the indexing model from the issue text.