cli: fix local source builds for rbwasm --ruby-version <dir> (#638) #764

Job	Run time
ci-data	4s
Push source gem	-1s
Compile native gem for aarch64-linux	5m 47s
Compile native gem for x64-mingw-ucrt	5m 45s
Compile native gem for aarch64-linux-musl	6m 7s
Compile native gem for x86_64-darwin	6m 2s
Compile native gem for x86_64-linux	8m 52s
Compile native gem for arm64-darwin	5m 56s
Compile native gem for x86_64-linux-musl	5m 44s
	44m 16s

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