Write gem files atomically #7541

Job	Run time
Rubygems on Ubuntu (3.4)	4m 1s
Rubygems on macOS (3.2)	2m 44s
Rubygems on Ubuntu (3.3)	2m 1s
Rubygems on macOS (3.4)	4m 57s
Rubygems on macOS (3.3)	1m 44s
Rubygems on Ubuntu (truffleruby)	3m 38s
Rubygems on Ubuntu (jruby)	7m 57s
Rubygems on Ubuntu (3.2)	2m 4s
Rubygems on Windows (3.2)	4m 16s
Rubygems on Windows (3.3)	4m 10s
Rubygems on Windows (3.4)	3m 48s
All rubygems jobs pass	5s
	41m 25s

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