fix #804: use xfun::file_rename() #664

Job	Run time
windows-latest (release) [Pandoc latest]	4m 15s
macOS-latest (release) [Pandoc latest]	1m 58s
ubuntu-latest (release) [Pandoc devel]	3m 25s
ubuntu-latest (release) [Pandoc 3.1.11]	3m 32s
ubuntu-latest (release) [Pandoc 2.19.2]	3m 14s
ubuntu-latest (release) [Pandoc 2.18]	3m 14s
ubuntu-latest (release) [Pandoc 2.17.1.1]	3m 11s
ubuntu-latest (release) [Pandoc 2.16.2]	3m 14s
ubuntu-latest (devel) [Pandoc 3.1.11]	11m 15s
ubuntu-latest (oldrel-1) [Pandoc 3.1.11]	3m 7s
ubuntu-latest (oldrel-2) [Pandoc 3.1.11]	3m 45s
ubuntu-latest (oldrel-3) [Pandoc 3.1.11]	3m 38s
	47m 48s

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