build: fix arm64 vcpkg triplet selection error (#18239)

microsoft/vcpkg-tool#1474 now validates that the target triplet is
valid. Unfortunately, `ARM64` is not valid... despite VS defaulting to
it.

VS 17.12 moved to the newer version of the vcpkg tool.

Given that we still want to build on VS 17.12, this commit adds a local
workaround.

See DD-2302065 for the internal tracking bug.
 
See microsoft/vcpkg#42182 for the upstream fix.

src/common.build.pre.props

@@ -275,6 +275,9 @@
     <VcpkgOSTarget>windows</VcpkgOSTarget>
     <VcpkgUseStatic>true</VcpkgUseStatic>
 
+    <!-- BODGY - work around DD#2302065. vcpkg now validates target triplets, and is *case sensitive* -->
+    <VcpkgPlatformTarget Condition="'$(Platform)'=='ARM64'">arm64</VcpkgPlatformTarget>
+
     <VcpkgAdditionalInstallOptions>--x-feature=terminal</VcpkgAdditionalInstallOptions>
     <!--
       Since we link everything statically, we don't need to copy anything.