run-task: clean up subprocess usage in _clean_git_checkout

This was copied from `run_command` in
8d7d8b1, but some stuff doesn't make
sense here:
- we don't stream the output to our log so we don't need to disable
  buffering or go through a TextIOWrapper
- we don't want to pass our stdin to the command

Instead we can use subprocess.run and let it capture stdout and check
the returncode.

Author: jcristau

src/taskgraph/run-task/run-task

@@ -597,26 +597,18 @@ def _clean_git_checkout(destination_path):
         "-nxdff",
     ]
     print_line(b"vcs", b"executing %r\n" % args)
-    p = subprocess.Popen(
+    p = subprocess.run(
         args,
-        # Disable buffering because we want to receive output
-        # as it is generated so timestamps in logs are
-        # accurate.
-        bufsize=0,
         stdout=subprocess.PIPE,
-        stdin=sys.stdin.fileno(),
+        encoding="latin1",
         cwd=destination_path,
         env=os.environ,
+        check=True,
     )
-    stdout = io.TextIOWrapper(p.stdout, encoding="latin1")
-    ret = p.wait()
-    if ret:
-        sys.exit(ret)
-    data = stdout.read()
     prefix = "Would remove "
     filenames = [
         os.path.join(destination_path, line[len(prefix) :])
-        for line in data.splitlines()
+        for line in p.stdout.splitlines()
     ]
     print_line(b"vcs", b"removing %r\n" % filenames)
     for filename in filenames: