[rubygems/rubygems] Improve install advice when some gems are not found

This problem is quite specific to our dev environment, but I guess the
fix could be handy for other situations.

After merging a change to treat default gems as regular gems, I get this
when trying to run `rubocop` on our repo:

```
$ bin/rubocop  --only Performance/RegexpMatch
Could not find json-2.6.3 in locally installed gems
Run `bundle install --gemfile /Users/deivid/code/rubygems/rubygems/tool/bundler/lint_gems.rb` to install missing gems.
```

However, when running the suggested command, nothing changes and I still
get the same error:

```
$ bundle install --gemfile /Users/deivid/code/rubygems/rubygems/tool/bundler/lint_gems.rb
Using ast 2.4.2
Using bundler 2.4.10
Using json 2.6.3
Using parallel 1.23.0
Using racc 1.7.1
Using parser 3.2.2.3
Using rainbow 3.1.1
Using regexp_parser 2.8.1
Using rexml 3.2.5
Using rubocop-ast 1.29.0
Using ruby-progressbar 1.13.0
Using unicode-display_width 2.4.2
Using rubocop 1.52.1
Using rubocop-performance 1.14.2
Bundle complete! 2 Gemfile dependencies, 14 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

$ bin/rubocop  --only Performance/RegexpMatch
Could not find json-2.6.3 in locally installed gems
Run `bundle install --gemfile /Users/deivid/code/rubygems/rubygems/tool/bundler/lint_gems.rb` to install missing gems.
```

The problem is that our `bin/rubocop` script uses the development
version of Bundler (which has the change causing the problem), but the
advice recommands the default version of Bundler, which does not yet
have the change.

This commit changes the advice to recommend to use the same version of
Bundler that run into the problem in the first place.

So in the above situation you now get:

```
$ bin/rubocop  --only Performance/RegexpMatch
Could not find json-2.6.3 in locally installed gems
Run `/Users/deivid/code/rubygems/rubygems/bundler/exe/bundle install --gemfile /Users/deivid/code/rubygems/rubygems/tool/bundler/lint_gems.rb` to install missing gems.
```

And running that fixes the problem:

```
$ /Users/deivid//rubygems/rubygems/bundler/exe/bundle install --gemfile /Users/deivid/code/rubygems/rubygems/tool/bundler/lint_gems.rb
Fetching gem metadata from https://rubygems.org/.........
Fetching json 2.6.3
Installing json 2.6.3 with native extensions
Bundle complete! 2 Gemfile dependencies, 14 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
```

rubygems/rubygems@10a9588c6d

lib/bundler/setup.rb

@@ -12,7 +12,8 @@
       Bundler.ui.error e.message
       Bundler.ui.warn e.backtrace.join("\n") if ENV["DEBUG"]
       if e.is_a?(Bundler::GemNotFound)
-        suggested_cmd = "bundle install"
+        suggested_bundle = Gem.loaded_specs["bundler"] ? "bundle" : Bundler::SharedHelpers.bundle_bin_path
+        suggested_cmd = "#{suggested_bundle} install"
         original_gemfile = Bundler.original_env["BUNDLE_GEMFILE"]
         suggested_cmd += " --gemfile #{original_gemfile}" if original_gemfile
         Bundler.ui.warn "Run `#{suggested_cmd}` to install missing gems."

lib/bundler/shared_helpers.rb

@@ -304,6 +304,13 @@ def set_env(key, value)
     public :set_env
 
     def set_bundle_variables
+      Bundler::SharedHelpers.set_env "BUNDLE_BIN_PATH", bundle_bin_path
+      Bundler::SharedHelpers.set_env "BUNDLE_GEMFILE", find_gemfile.to_s
+      Bundler::SharedHelpers.set_env "BUNDLER_VERSION", Bundler::VERSION
+      Bundler::SharedHelpers.set_env "BUNDLER_SETUP", File.expand_path("setup", __dir__)
+    end
+
+    def bundle_bin_path
       # bundler exe & lib folders have same root folder, typical gem installation
       exe_file = File.expand_path("../../exe/bundle", __dir__)
 
@@ -313,11 +320,9 @@ def set_bundle_variables
       # bundler is a default gem, exe path is separate
       exe_file = Bundler.rubygems.bin_path("bundler", "bundle", VERSION) unless File.exist?(exe_file)
 
-      Bundler::SharedHelpers.set_env "BUNDLE_BIN_PATH", exe_file
-      Bundler::SharedHelpers.set_env "BUNDLE_GEMFILE", find_gemfile.to_s
-      Bundler::SharedHelpers.set_env "BUNDLER_VERSION", Bundler::VERSION
-      Bundler::SharedHelpers.set_env "BUNDLER_SETUP", File.expand_path("setup", __dir__)
+      exe_file
     end
+    public :bundle_bin_path
 
     def set_path
       validate_bundle_path