[rubygems/rubygems] Don't show bug report template when GEM_HOME has no writable bit

Instead, don't check that at all and proceed. If something fails to be
written inside GEM_HOME, we'll eventually fail with a proper permissions
error.

In addition to that, the writable bit in GEM_HOME is not even reliable,
because only the immediate parent is actually checked when writing. For
example,

```
$ mkdir -p foo/bar
$ chmod -w foo
$ touch foo/bar/baz # writes without issue
```

https://github.com/rubygems/rubygems/commit/4bced7ac73

lib/bundler/rubygems_gem_installer.rb

@@ -45,6 +45,14 @@ module Bundler
       spec
     end
 
+    def pre_install_checks
+      super
+    rescue Gem::FilePermissionError
+      # Ignore permission checks in RubyGems. Instead, go on, and try to write
+      # for real. We properly handle permission errors when they happen.
+      nil
+    end
+
     def generate_plugins
       return unless Gem::Installer.instance_methods(false).include?(:generate_plugins)
 

spec/bundler/commands/install_spec.rb

@@ -792,6 +792,41 @@ RSpec.describe "bundle install with gem sources" do
     end
   end
 
+  describe "when gem home does not have the writable bit set, yet it's still writable", :permissions do
+    let(:gem_home) { bundled_app("vendor/#{Bundler.ruby_scope}") }
+
+    before do
+      build_repo4 do
+        build_gem "foo", "1.0.0" do |s|
+          s.write "CHANGELOG.md", "foo"
+        end
+      end
+
+      gemfile <<-G
+        source "#{file_uri_for(gem_repo4)}"
+        gem 'foo'
+      G
+    end
+
+    it "should display a proper message to explain the problem" do
+      bundle "config set --local path vendor"
+      bundle :install
+      expect(out).to include("Bundle complete!")
+      expect(err).to be_empty
+
+      FileUtils.chmod("-w", gem_home)
+
+      begin
+        bundle "install --redownload"
+      ensure
+        FileUtils.chmod("+w", gem_home)
+      end
+
+      expect(out).to include("Bundle complete!")
+      expect(err).to be_empty
+    end
+  end
+
   describe "when bundle cache path does not have write access", :permissions do
     let(:cache_path) { bundled_app("vendor/#{Bundler.ruby_scope}/cache") }