Microsoft.Build.Internal.EngineFileUtilities.GetFileListEscaped is 3x slower for netcore projects

[arunchndr]

Child of #6034. Traces at "\SLNATSCALE002\CoreVsFramework\Traces\Core_400.etl.zip"

For 400 projects it takes 960ms(netcore) vs 320ms(netframework) and the delta increases with project.

[ladipro]

Likely addressed with #6151, will confirm.

[ladipro]

Acceptance criteria: The cost of GetFileListEscaped is reduced by at least 20%.