VS: Fix VCTargetsPath detection when cross-compiling

Tell `MSBuild` to use the same `Platform` that we generate into the
`VCTargetsPath` detection project.

Fixes: #22068
48 jobs for !6053 with vs-VCTargetsPath-Platform in 48 minutes and 31 seconds (queued for 7 seconds)
latest detached
Status Job ID Name Coverage
  Build
manual #5741464
linux docker build cmake manual
build:centos6-x86_64
passed #5741461
linux docker build cmake
build:debian10-iwyu

00:02:28

manual #5741465
cmake linux-3.17 linux docker build manual
build:fedora33-ninja
passed #5741463
linux-3.17 linux docker build cmake
build:fedora33-sphinx

00:04:59

passed #5741462
linux-3.17 linux docker build cmake
build:fedora33-tidy

00:06:24

manual #5741467
nonconcurrent xcode-12.4 shell macos cmake manual
build:macos-x86_64-makefiles
manual #5741466
nonconcurrent xcode-12.4 shell macos cmake manual
build:macos-x86_64-ninja
passed #5741468
nonconcurrent msvc-19.28-16.9 vs2019 shell windows cmake manual
build:windows-vs2019-x64-ninja

00:09:37

 
  Test
created #5741469
x11 linux-3.17 linux docker cmake
test:fedora33-ninja
created #5741471
nonconcurrent xcode-12.4 shell macos cmake
test:macos-x86_64-makefiles
created #5741470
nonconcurrent xcode-12.4 shell macos cmake
test:macos-x86_64-ninja
passed #5741472
nonconcurrent msvc-19.28-16.9 vs2019 shell windows cmake
test:windows-vs2019-x64-ninja

00:05:50

 
  Test Ext
created #5741475
linux docker cuda-rt cmake
test:cuda10.2-nvidia
created #5741473
linux-3.17 linux docker build cmake
test:debian10-ninja
created #5741474
linux-3.17 linux docker build cmake
test:fedora33-makefiles
created #5741476
linux-3.17 linux docker build cmake
test:fedora33-ninja-multi
created #5741477
concurrent xcode-12.4 shell macos cmake
test:macos-x86_64-xcode
passed #5741478
concurrent msvc-19.28-16.9 vs2019 shell windows cmake
test:windows-vs2019-x64

00:32:31

 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-stager

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passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

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passed ghostflow-stager

passed ghostflow-stager

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