meta: remove let from for loops

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

lib

Description of change

This is a known de-opt. It may not be 100% necessary in all cases but it seems like a decent enough idea to avoid it.

If individuals are interested we can create a linting rule for this