Enable easy passing of environment variables in `browser.script.iffe.js` #780

Job	Run time
ci-data	4s
Push source gem	-1s
Compile native gem for aarch64-linux-musl	6m 14s
Compile native gem for x86_64-linux-musl	6m 58s
Compile native gem for x64-mingw-ucrt	6m 31s
Compile native gem for aarch64-linux	6m 58s
Compile native gem for arm64-darwin	6m 35s
Compile native gem for x86_64-darwin	6m 46s
Compile native gem for x86_64-linux	10m 58s
	51m 3s

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