GNUmakefile: prefer system {ln,install}.exe #13237

Job	Run time
Style/format (ubuntu-latest, feat_os_unix)	20s
Style/lint (ubuntu-latest, all, true)	1m 14s
Style/lint (windows-latest, feat_os_windows)	1m 56s
Style/lint (macos-latest, feat_os_macos)	1m 27s
Style/spelling (ubuntu-latest, feat_os_unix)	1m 6s
Style/toml	13s
Style/Python	7s
Pre-commit hooks	39s
	7m 2s

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