Use download cache also for tools

[autoantwort]

There is the option --only-downloads to download all files necessary to build the package. You can copy the complete dir to another machine that don't has internet as proposed in #16391. But if you only copy the compressed archives and not the files in tools/*, vcpkg tries to download these files again even though they are already there.
I propose that vcpkg should not download the archives for the tools again when the archives are already there.

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Is your feature request related to a problem? Please describe.
You have to copy a lot more files if you also have to copy the tools folder.

Proposed solution
vcpkg should not download the archives for the tools again when the archives are already there.

Describe alternatives you've considered
I have no alternative in mind

Additional context
We want to put the downloads folder also in our own copy of vcpkg (in git) so that we can compile even if we have no internet or when some downloads mirror is offline. Its would be much nicer if we can check in 6 archives instead of over 19'000 files.