Fix subtraction logic on non-input cache policies

[cicdw]

This PR implements @a14e's suggested bug fix as discussed here.

In particular, cache policies which do not already rely on run inputs should not implicitly begin relying on run inputs just because the subtraction operator was used.

[codspeed-hq]

CodSpeed Performance Report

Merging #16840 will not alter performance

_{Comparing fix-cache-policy-subtraction (c674e62) with main (ff0f3db)}

Summary

✅ 2 untouched benchmarks

[zzstoatzz]

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