Gemfile: Avoid installing rdoc in TruffleRuby. #909

Job	Run time
openssl-1.0.2u	2m 21s
openssl-1.1.0l	1m 28s
openssl-1.1.1w	1m 39s
openssl-3.0.12	2m 46s
openssl-3.1.4	2m 49s
openssl-3.2.0	3m 42s
libressl-3.1.5	2m 4s
libressl-3.2.7	2m 18s
libressl-3.3.6	2m 7s
libressl-3.4.3	2m 8s
libressl-3.5.3	2m 28s
libressl-3.6.3	2m 13s
libressl-3.7.3	2m 12s
libressl-3.8.1	2m 10s
openssl-3.0.12 fips	2m 38s
openssl-3.1.4 fips	2m 43s
openssl-3.2.0 fips	3m 19s
openssl-head	3m 32s
openssl-head fips	3m 23s
ruby-versions / Generate Ruby versions	3s
ubuntu-22.04 2.7	56s
ubuntu-22.04 3.0	53s
ubuntu-22.04 3.1	56s
ubuntu-22.04 3.2	56s
ubuntu-22.04 head	56s
ubuntu-22.04 truffleruby	1m 27s
ubuntu-22.04 truffleruby-head	1m 32s
ubuntu-20.04 2.7	51s
ubuntu-20.04 3.0	47s
ubuntu-20.04 3.1	48s
ubuntu-20.04 3.2	46s
ubuntu-20.04 head	51s
ubuntu-20.04 truffleruby-head	1m 20s
macos-latest 2.7	1m 19s
macos-latest 3.0	1m 18s
macos-latest 3.1	52s
macos-latest 3.2	1m 0s
macos-latest head	1m 36s
macos-latest truffleruby-head	3m 4s
windows-latest 2.7	4m 19s
windows-latest 3.0	3m 58s
windows-latest 3.1	3m 54s
windows-latest 3.2	6m 14s
windows-latest ucrt	3m 35s
windows-latest mswin	2m 16s
	1h 34m 27s

Provide feedback