Skip TestCommitEmail if git is not available.

https://github.com/ruby/actions/actions/runs/19420968008/job/55558315338

```
    1) Error:
  TestCommitEmail#test_sendmail_encoding:
  Test::Unit::ProxyError: No such file or directory - git
      /home/runner/work/actions/actions/ruby-4.0.0-preview2/lib/open3.rb:534:in 'Kernel#spawn'
      /home/runner/work/actions/actions/ruby-4.0.0-preview2/lib/open3.rb:534:in 'Open3.popen_run'
      /home/runner/work/actions/actions/ruby-4.0.0-preview2/lib/open3.rb:379:in 'Open3.popen2'
      /home/runner/work/actions/actions/ruby-4.0.0-preview2/lib/open3.rb:785:in 'Open3.capture2'
      /home/runner/work/actions/actions/ruby-4.0.0-preview2/tool/test/test_commit_email.rb:89:in 'TestCommitEmail#git'
      /home/runner/work/actions/actions/ruby-4.0.0-preview2/tool/test/test_commit_email.rb:13:in 'block in TestCommitEmail#setup'
      /home/runner/work/actions/actions/ruby-4.0.0-preview2/tool/test/test_commit_email.rb:12:in 'Dir.chdir'
      /home/runner/work/actions/actions/ruby-4.0.0-preview2/tool/test/test_commit_email.rb:12:in 'TestCommitEmail#setup'

    2) Error:
  TestCommitEmail#test_sendmail_encoding:
  Test::Unit::ProxyError: no implicit conversion of nil into String
      /home/runner/work/actions/actions/ruby-4.0.0-preview2/tool/test/test_commit_email.rb:37:in 'File.unlink'
      /home/runner/work/actions/actions/ruby-4.0.0-preview2/tool/test/test_commit_email.rb:37:in 'TestCommitEmail#teardown'
```

Author: hsbt

tool/test/test_commit_email.rb

@@ -8,6 +8,8 @@ class TestCommitEmail < Test::Unit::TestCase
   STDIN_DELIMITER = "---\n"
 
   def setup
+    omit 'git command is not available' unless system('git', '--version', out: File::NULL, err: File::NULL)
+
     @ruby = Dir.mktmpdir
     Dir.chdir(@ruby) do
       git('init', '--initial-branch=master')