win: replace alloca() with stack-based array
win: replace alloca() with stack-based array

`required_vars_value_len` has a fixed number of elements. There is no
need to use alloca() to allocate it.

Refs: https://github.com/libuv/libuv/pull/2781
PR-URL: https://github.com/libuv/libuv/pull/2783
Reviewed-By: Santiago Gimeno <3f17e29e343c4cf8809fb0970d81d6d8ee88d6b3@gmail.com>
Reviewed-By: Richard Lau <8193d74c06330af7778ef55510160e4e561ed6a1@uk.ibm.com>
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>

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