Use env var to set api key on install

This commit allows setting the authorization header for `gem install`
commands using the GEM_HOST_API_KEY environment variable. This is useful
when working with private registries using short lived api tokens (e.g.,
OIDC tokens). For example:

```
GEM_HOST_API_KEY="secret_token" gem install private_gem --clear-sources
\ --source https://private.repo.com
```

The GEM_HOST_API_KEY environment variable can already be used by the
`gem push` command. I'm simply extending it's usage to `gem install` as
well.

Author: swebb

lib/rubygems/commands/install_command.rb

@@ -159,6 +159,10 @@ def execute
 
     load_hooks
 
+    if ENV["GEM_HOST_API_KEY"]
+      Gem::RemoteFetcher.fetcher.headers[:authorization] = ENV["GEM_HOST_API_KEY"]
+    end
+
     exit_code = install_gems
 
     show_installed

test/rubygems/test_gem_commands_install_command.rb

@@ -28,6 +28,10 @@ def teardown
     Gem::Command.build_args = @orig_args
     File.unlink @gemdeps if File.file? @gemdeps
     File.unlink "#{@gemdeps}.lock" if File.file? "#{@gemdeps}.lock"
+
+    if defined? Gem::RemoteFetcher
+      Gem::RemoteFetcher.fetcher = nil
+    end
   end
 
   def test_execute_exclude_prerelease
@@ -760,6 +764,22 @@ def test_execute_remote
     assert_match "1 gem installed", @ui.output
   end
 
+  def test_execute_remote_gem_host_api_key
+    Gem::RemoteFetcher.fetcher = @fetcher = Gem::FakeFetcher.new
+    @a1, @a1_gem = util_gem "a", "1"
+    util_setup_spec_fetcher @a1
+    @fetcher.data["http://gems.example.com/gems/a-1.gem"] = Gem.read_binary(@a1_gem)
+
+    ENV["GEM_HOST_API_KEY"] = "temporary_secret_key"
+    @cmd.options[:args] = %w[a]
+    use_ui @stub_ui do
+      @cmd.execute
+    end
+
+    assert_match "1 gem installed", @ui.output
+    assert_equal "temporary_secret_key", @fetcher.headers[:authorization]
+  end
+
   def test_execute_with_invalid_gem_file
     FileUtils.touch("a.gem")
 

test/rubygems/utilities.rb

@@ -31,11 +31,13 @@
 
 class Gem::FakeFetcher
   attr_reader :data
+  attr_reader :headers
   attr_reader :last_request
   attr_accessor :paths
 
   def initialize
     @data = {}
+    @headers = {}
     @paths = []
   end