Install files in parallel, redux

[BillyONeal]

Alternative to: #1256

I created a test port that just writes 10'000 files:

set(INCLUDE_DIR "${CURRENT_PACKAGES_DIR}/include/${PORT}")
file(MAKE_DIRECTORY "${INCLUDE_DIR}")

foreach(i RANGE 1 10000)
    set(header_name "header${i}")
    file(WRITE "${INCLUDE_DIR}/${header_name}.h"
"// Generated header ${header_name}
#pragma once
constexpr int ${header_name}_value = ${i};
")
endforeach()

file(MAKE_DIRECTORY "${CURRENT_PACKAGES_DIR}/share/${PORT}")
vcpkg_install_copyright(FILE_LIST "${CMAKE_CURRENT_LIST_DIR}/copyright")

Testing before/after this change for vcpkg install testing-headers shows a meaningful performance improvement; I'm guessing due to antivirus involvement or similar

This is on AMD RYZEN AI MAX+ PRO 395 w/ Radeon 8060S

[BillyONeal]

GitHub's diff is awful. This line should go with 185 on the 'red' side.

[BillyONeal]

Obligatory cult of Beyond Compare recruitment flyer:

[vicroms]

LGTM