x509attr: avoid using OpenSSL::ASN1 internals in #value= #1105

Job	Run time
openssl-1.0.2u	2m 26s
openssl-1.1.0l	1m 36s
openssl-1.1.1w	1m 51s
openssl-3.0.13	2m 48s
openssl-3.1.5	2m 54s
openssl-3.2.1	3m 13s
openssl-3.3.0	3m 27s
libressl-3.1.5	2m 8s
libressl-3.2.7	2m 15s
libressl-3.3.6	2m 22s
libressl-3.4.3	2m 14s
libressl-3.5.3	2m 23s
libressl-3.6.3	2m 20s
libressl-3.7.3	2m 24s
libressl-3.8.4	2m 14s
libressl-3.9.1	2m 18s
openssl-3.0.13 fips	2m 45s
openssl-3.1.5 fips	2m 45s
openssl-3.2.1 fips	3m 13s
openssl-3.3.0 fips	3m 12s
openssl-head	3m 18s
openssl-head fips	3m 17s
openssl-head no-legacy	3m 20s
ruby-versions / Generate Ruby versions	4s
ubuntu-22.04 2.7	1m 3s
ubuntu-22.04 3.0	57s
ubuntu-22.04 3.1	1m 1s
ubuntu-22.04 3.2	1m 1s
ubuntu-22.04 3.3	1m 1s
ubuntu-22.04 head	59s
ubuntu-22.04 truffleruby	1m 34s
ubuntu-22.04 truffleruby-head	1m 45s
ubuntu-20.04 2.7	53s
ubuntu-20.04 3.0	56s
ubuntu-20.04 3.1	57s
ubuntu-20.04 3.2	51s
ubuntu-20.04 3.3	53s
ubuntu-20.04 head	52s
ubuntu-20.04 truffleruby-head	1m 30s
macos-latest 2.7	51s
macos-latest 3.0	44s
macos-latest 3.1	41s
macos-latest 3.2	45s
macos-latest 3.3	48s
macos-latest head	47s
macos-latest truffleruby-head	59s
windows-latest 2.7	6m 9s
windows-latest 3.0	4m 2s
windows-latest 3.1	3m 44s
windows-latest 3.2	4m 10s
windows-latest 3.3	4m 2s
windows-latest head	3m 59s
windows-latest ucrt	3m 29s
windows-latest mswin	3m 15s
	1h 55m 25s

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