[rubygems/rubygems] Better suggestion when `bundler/setup` fails due to missing gems

If the original `BUNDLE_GEMFILE` is different from the default, then the
suggestion wouldn't work as is.

Before:

```
$ util/rubocop
Could not find rubocop-1.30.1 in locally installed gems
Run `bundle install` to install missing gems.

$  rubygems git:(better-cmd-suggestion) ✗ bundle install
Could not locate Gemfile
```

After:

```
$ util/rubocop
Could not find rubocop-1.30.1 in locally installed gems
Run `bundle install --gemfile /path/to/rubygems/bundler/tool/bundler/lint_gems.rb` to install missing gems.

$ bundle install --gemfile /path/to/rubygems/bundler/tool/bundler/lint_gems.rb
Fetching gem metadata from https://rubygems.org/.........
Using ast 2.4.2
Using bundler 2.4.7
Using parser 3.1.2.0
Using rainbow 3.1.1
Using parallel 1.22.1
Using regexp_parser 2.5.0
Using rubocop-ast 1.18.0
Using rexml 3.2.5
Using ruby-progressbar 1.11.0
Using unicode-display_width 2.1.0
Fetching rubocop 1.30.1
Installing rubocop 1.30.1
Using rubocop-performance 1.14.2
Bundle complete! 2 Gemfile dependencies, 12 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

$ util/rubocop
Inspecting 345 files
.........................................................................................................................................................................................................................................................................................................................................................

345 files inspected, no offenses detected
```

https://github.com/rubygems/rubygems/commit/bf1320d805

1 files changed, 24 insertions, 0 deletions

diff --git a/spec/bundler/commands/exec_spec.rb b/spec/bundler/commands/exec_spec.rb
index 5ca11dcba2..099cdb39fa 100644
--- a/spec/bundler/commands/exec_spec.rb
+++ b/spec/bundler/commands/exec_spec.rb
@@ -916,6 +916,30 @@ Run `bundle install` to install missing gems.
       end
     end
 
+    context "when Bundler.setup fails and Gemfile is not the default" do
+      before do
+        create_file "CustomGemfile", <<-G
+          source "#{file_uri_for(gem_repo1)}"
+          gem 'rack', '2'
+        G
+        ENV["BUNDLER_FORCE_TTY"] = "true"
+        ENV["BUNDLE_GEMFILE"] = "CustomGemfile"
+        ENV["BUNDLER_ORIG_BUNDLE_GEMFILE"] = nil
+      end
+
+      let(:exit_code) { Bundler::GemNotFound.new.status_code }
+      let(:expected) { "" }
+
+      it "prints proper suggestion" do
+        skip "https://github.com/rubygems/rubygems/issues/3351" if Gem.win_platform?
+
+        subject
+        expect(exitstatus).to eq(exit_code)
+        expect(err).to include("Run `bundle install --gemfile CustomGemfile` to install missing gems.")
+        expect(out).to eq(expected)
+      end
+    end
+
     context "when the executable exits non-zero via at_exit" do
       let(:executable) { super() + "\n\nat_exit { $! ? raise($!) : exit(1) }" }
       let(:exit_code) { 1 }