Apply system Python filtering to executable name requests

[zanieb]

Executable name requests were being treated as explicit requests to install into system environments, but I don't think it should be as it's implicit what environment you'll end up in. Following #4308, we allow multiple executables to be found so we can filter here.

Concretely, this means --system is required to install into a system environment discovered with e.g. --python=python. The flag is still not required for cases where we're not mutating environment.

[codspeed-hq]

CodSpeed Performance Report

Merging #4309 will improve performances by 10.39%

_{Comparing zb/executable-name-system (85bf490) with release/3 (3eefac4)}

Summary

⚡ 1 improvements
✅ 13 untouched benchmarks

Benchmarks breakdown

	Benchmark	release/3	zb/executable-name-system	Change
⚡	wheelname_tag_compatibility[flyte-long-compatible]	2.2 µs	2 µs	+10.39%