Fix bundle exec rake install not working

These gem task checks for a specific string in the gem subcommand
output. However, if we are inside a bundled environment (`bundle exec
rake install` instead of `rake install`), the ruby process will require
`bundler/setup` first thing, and initialize a silent UI for rubygems. As
a result, the output string will be always empty and the condition for
success will always fail. So, even if installation succeeded, user will
always be notified of a failure like:

```
foo 1.0 built to pkg/foo-1.0.gem.
rake aborted!
Couldn't install gem, run `gem install /home/deivid/Code/bundler/tmp/1/bundled_app/pkg/foo-1.0.gem' for more detailed output
...
```

lib/bundler/gem_helper.rb

@@ -86,7 +86,7 @@ def install_gem(built_gem_path = nil, local = false)
       built_gem_path ||= build_gem
       cmd = "#{gem_command} install #{built_gem_path}"
       cmd += " --local" if local
-      out, status = sh_with_status(cmd.shellsplit)
+      out, status = Bundler.with_original_env { sh_with_status(cmd.shellsplit) }
       unless status.success? && out[/Successfully installed/]
         raise "Couldn't install gem, run `gem install #{built_gem_path}' for more detailed output"
       end

spec/runtime/gem_tasks_spec.rb

@@ -6,15 +6,25 @@
       f.write <<-GEMSPEC
         Gem::Specification.new do |s|
           s.name = "foo"
+          s.version = "1.0"
+          s.summary = "dummy"
+          s.author = "Perry Mason"
         end
       GEMSPEC
     end
+
     bundled_app("Rakefile").open("w") do |f|
       f.write <<-RAKEFILE
         $:.unshift("#{lib_dir}")
         require "bundler/gem_tasks"
       RAKEFILE
     end
+
+    install_gemfile! <<-G
+      source "#{file_uri_for(gem_repo1)}"
+
+      gem "rake"
+    G
   end
 
   it "includes the relevant tasks" do
@@ -35,6 +45,18 @@
     expect(exitstatus).to eq(0) if exitstatus
   end
 
+  it "defines a working `rake install` task" do
+    with_gem_path_as(Spec::Path.base_system_gems.to_s) do
+      sys_exec "#{rake} install", "RUBYOPT" => "-I#{lib_dir}"
+    end
+
+    expect(err).to be_empty
+
+    bundle! "exec rake install"
+
+    expect(err).to be_empty
+  end
+
   it "adds 'pkg' to rake/clean's CLOBBER" do
     with_gem_path_as(Spec::Path.base_system_gems.to_s) do
       sys_exec! %(#{rake} -e 'load "Rakefile"; puts CLOBBER.inspect')