perf: download and save mkcert in stream (#63527)
Previously, Next.js would buffer the `mkcert` binary in memory and write
it to disk all at once. The pull request changes this to pipe the
download stream to the disk. This improves memory consumption by
avoiding having to load the entire `mkcert` binary into memory.
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Co-authored-by: JJ Kasper <jj@jjsweb.site>
