macOS

Expand `$destdir` to enable rdoc plugins for rubygems #4353

Job	Run time
make (check, macos-14)	6m 36s
make (check, macos-14, --with-gcc=gcc-14)	8m 4s
make (check, macos-14, --with-jemalloc --with-opt-dir=$(brew --prefix jemalloc))	6m 35s
make (check, macos-14, --with-gmp)	7m 2s
make (test-all, --repeat-count=2, macos-14)	9m 19s
make (test-bundler-parallel, macos-14)	20m 33s
make (test-bundled-gems, macos-14)	10m 7s
make (check, macos-15)	5m 7s
make (check, macos-13)	10m 51s
macOS result	1s
	1h 24m 15s

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