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 Name Job ID Coverage
  Build
manual build:centos6-x86_64 #5741464
cmake build docker linux manual
passed build:debian10-iwyu #5741461
cmake build docker linux

00:02:28

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

00:04:59

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

00:06:24

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

00:09:37

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

00:05:50

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

00:32:31

 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-stager

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