weakref.gemspec: Drop gemspec config on executables #54

Job	Run time
ruby-versions / Generate Ruby versions	2s
build (2.4 / ubuntu-latest)	18s
build (2.5 / ubuntu-latest)	18s
build (2.6 / ubuntu-latest)	19s
build (2.6 / macos-latest)	25s
build (2.7 / ubuntu-latest)	24s
build (2.7 / macos-latest)	27s
build (3.0 / ubuntu-latest)	9s
build (3.0 / macos-latest)	9s
build (3.1 / ubuntu-latest)	6s
build (3.1 / macos-latest)	8s
build (3.2 / ubuntu-latest)	9s
build (3.2 / macos-latest)	6s
build (3.3 / ubuntu-latest)	5s
build (3.3 / macos-latest)	10s
build (3.4 / ubuntu-latest)	5s
build (3.4 / macos-latest)	8s
build (head / ubuntu-latest)	6s
build (head / macos-latest)	9s
	3m 43s

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