Move remaining undef use in OS extensions to prepend

[dduugg]

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Relates to #17998

Opening this as one big mess in draft form for folks to consider, very open to breaking this into digestible chunks.

I recommend hiding whitespace when reviewing.

[MikeMcQuaid]

Makes sense to me! Good to go when 🟢

[dduugg]

(This is still on my radar, I anticipate working on it next week)

[dduugg]

Reverting the ENV changes resolves the Ubuntu 20.04 failure. Going to merge the rest now per @MikeMcQuaid's blessing above. I'll work on the ENV piece separately and open another PR for that one.