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 build:centos6-x86_64 #5741464
linux docker build cmake manual
passed build:debian10-iwyu #5741461
linux docker build cmake

00:02:28

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

00:04:59

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

00:06:24

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

00:09:37

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

00:05:50

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

00:32:31

 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-stager

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passed ghostflow-stager

passed ghostflow-stager

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