GNUmakefile: prefer system {ln,install}.exe #13234

Job	Run time
Style/lint (windows-latest, feat_os_windows)	2m 33s
Style/lint (macos-latest, feat_os_macos)	1m 30s
Style/format (ubuntu-latest, feat_os_unix)	18s
Style/lint (ubuntu-latest, all, true)	1m 15s
Style/spelling (ubuntu-latest, feat_os_unix)	2m 17s
Style/toml	16s
Pre-commit hooks	44s
Style/Python	5s
	8m 58s

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