Enable easy passing of environment variables in `browser.script.iffe.js` #782

Job	Run time
ci-data	5s
Push source gem	-1s
Compile native gem for x86_64-darwin	6m 51s
Compile native gem for x86_64-linux	11m 28s
Compile native gem for aarch64-linux	6m 4s
Compile native gem for aarch64-linux-musl	6m 47s
Compile native gem for x64-mingw-ucrt	6m 43s
Compile native gem for arm64-darwin	6m 38s
Compile native gem for x86_64-linux-musl	6m 44s
	51m 19s

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