Gemfile: Avoid installing rdoc in TruffleRuby. #910

Job	Run time
openssl-1.0.2u	2m 21s
openssl-1.1.0l	1m 27s
openssl-1.1.1w	1m 37s
openssl-3.0.12	2m 55s
openssl-3.1.4	2m 53s
openssl-3.2.0	3m 26s
libressl-3.1.5	2m 13s
libressl-3.2.7	2m 10s
libressl-3.3.6	2m 9s
libressl-3.4.3	2m 17s
libressl-3.5.3	2m 13s
libressl-3.6.3	2m 18s
libressl-3.7.3	2m 24s
libressl-3.8.1	2m 10s
openssl-3.0.12 fips	2m 48s
openssl-3.1.4 fips	3m 0s
openssl-3.2.0 fips	3m 25s
openssl-head	3m 48s
openssl-head fips	3m 27s
ruby-versions / Generate Ruby versions	1s
ubuntu-22.04 2.7	55s
ubuntu-22.04 3.0	51s
ubuntu-22.04 3.1	55s
ubuntu-22.04 3.2	53s
ubuntu-22.04 head	1m 0s
ubuntu-22.04 truffleruby	1m 25s
ubuntu-22.04 truffleruby-head	1m 27s
ubuntu-20.04 2.7	54s
ubuntu-20.04 3.0	47s
ubuntu-20.04 3.1	48s
ubuntu-20.04 3.2	55s
ubuntu-20.04 head	52s
ubuntu-20.04 truffleruby-head	1m 26s
macos-latest 2.7	1m 22s
macos-latest 3.0	1m 19s
macos-latest 3.1	57s
macos-latest 3.2	1m 19s
macos-latest head	53s
macos-latest truffleruby-head	2m 3s
windows-latest 2.7	4m 34s
windows-latest 3.0	4m 28s
windows-latest 3.1	3m 58s
windows-latest 3.2	5m 12s
windows-latest ucrt	3m 39s
windows-latest mswin	2m 12s
	1h 34m 6s

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