Fix return value for Ruby < 2.6

`FileUtils.cd` only returns the value of the block in Ruby >= 2.6.

lib/thor/actions.rb

@@ -181,12 +181,13 @@ def inside(dir = "", config = {}, &block)
         FileUtils.mkdir_p(destination_root)
       end
 
-      result = if pretend
+      result = nil
+      if pretend
         # In pretend mode, just yield down to the block
-        block.arity == 1 ? yield(destination_root) : yield
+        result = block.arity == 1 ? yield(destination_root) : yield
       else
         require "fileutils"
-        FileUtils.cd(destination_root) { block.arity == 1 ? yield(destination_root) : yield }
+        FileUtils.cd(destination_root) { result = block.arity == 1 ? yield(destination_root) : yield }
       end
 
       @destination_stack.pop

spec/actions_spec.rb

@@ -174,6 +174,10 @@ def file
         runner.inside("bar", :pretend => true) {}
         expect(File.exist?("bar")).to be false
       end
+
+      it "returns the value yielded by the block" do
+        expect(runner.inside("foo") { 123 }).to eq(123)
+      end
     end
 
     describe "when verbose" do