Fix loading of digest ext libraries with `--with-static-linked-ext` #1205

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-22.04 2.7	40s
ubuntu-22.04 3.0	17s
ubuntu-22.04 3.1	22s
ubuntu-22.04 3.2	21s
ubuntu-22.04 3.3	21s
ubuntu-22.04 head	20s
ubuntu-20.04 2.7	46s
ubuntu-20.04 3.0	19s
ubuntu-20.04 3.1	19s
ubuntu-20.04 3.2	25s
ubuntu-20.04 3.3	18s
ubuntu-20.04 head	18s
macos-14 2.7	46s
macos-14 3.0	19s
macos-14 3.1	25s
macos-14 3.2	26s
macos-14 3.3	27s
macos-14 head	22s
macos-13 2.7	1m 51s
macos-13 3.0	41s
macos-13 3.1	35s
macos-13 3.2	30s
macos-13 3.3	48s
macos-13 head	50s
windows-latest 2.7	4m 5s
windows-latest 3.0	2m 14s
windows-latest 3.1	2m 55s
windows-latest 3.2	3m 51s
windows-latest 3.3	2m 32s
windows-latest ucrt	2m 47s
windows-latest mswin	2m 0s
ubuntu-20.04 jruby	1m 23s
ubuntu-20.04 jruby-head	1m 26s
	36m 3s

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