Fix bundle lock regression when using update and lockfile flags:

- Ref #8917
- ### Problem

  Prior to Bundler 2.5.6, running
  `bundle lock --update foo --lockfile Gemfile_bumped.lock` would
  update only the foo gem and write the lockfile to the
  `Gemfile_bumped.lock`.

  In Bundler 2.5.6 and above running the same command, updates
  absolutely all gems.
  This change is related to #7047

  ### Solution

  Build the definition using the existing lockfile, not the one passed
  in the `--lockfile` as it may not exist. Dump the definition into
  a new lockfile.

Author: Edouard-chin

bundler/lib/bundler/cli/lock.rb

@@ -35,11 +35,8 @@ def run
         update = { bundler: bundler }
       end
 
-      file = options[:lockfile]
-      file = file ? Pathname.new(file).expand_path : Bundler.default_lockfile
-
       Bundler.settings.temporary(frozen: false) do
-        definition = Bundler.definition(update, file)
+        definition = Bundler.definition(update, Bundler.default_lockfile)
         definition.add_checksums if options["add-checksums"]
 
         Bundler::CLI::Common.configure_gem_version_promoter(definition, options) if options[:update]
@@ -71,8 +68,11 @@ def run
         if print
           puts definition.to_lock
         else
+          file = options[:lockfile]
+          file = file ? Pathname.new(file).expand_path : Bundler.default_lockfile
+
           puts "Writing lockfile to #{file}"
-          definition.lock
+          definition.lock(false, file)
         end
       end
 

bundler/spec/commands/lock_spec.rb

@@ -311,6 +311,44 @@
     expect { read_lockfile }.to raise_error(Errno::ENOENT)
   end
 
+  it "updates a specific gem and write to a custom location" do
+    build_repo4 do
+      build_gem "uri", %w[1.0.2 1.0.3]
+      build_gem "warning", %w[1.4.0 1.5.0]
+    end
+
+    gemfile <<~G
+      source "https://gem.repo4"
+
+      gem "uri"
+      gem "warning"
+    G
+
+    lockfile <<~L
+      GEM
+        remote: https://gem.repo4
+        specs:
+          uri (1.0.2)
+          warning (1.4.0)
+
+      PLATFORMS
+        #{lockfile_platforms}
+
+      DEPENDENCIES
+        uri
+        warning
+
+      BUNDLED WITH
+         #{Bundler::VERSION}
+    L
+
+    bundle "lock --update uri --lockfile=lock"
+
+    lockfile_content = read_lockfile("lock")
+    expect(lockfile_content).to include("uri (1.0.3)")
+    expect(lockfile_content).to include("warning (1.4.0)")
+  end
+
   it "writes to custom location using --lockfile when a default lockfile is present" do
     gemfile_with_rails_weakling_and_foo_from_repo4