Add %ProgramW6432% to the list of search paths when looking for 7-Zip.

This allows a 64bit 7-Zip installation to be located even though
win-setup.ps1 is run by a 32-bit process.

This applies to 64bit Windows (7, 10, Server 2008 R2, Server 2012).
Tested on 2012.

Ref:
https://msdn.microsoft.com/en-us/library/windows/desktop/aa384274%28v=vs.85%29.aspx

Change-Id: I6f4f3226b25c890cd674bf4c4d9ea73ddfc8ece0
Reviewed-on: https://code.wireshark.org/review/12740
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>

tools/win-setup.ps1

@@ -241,6 +241,7 @@ function Bootstrap7Zip() {
     $searchDirs = @(
         "${env:ProgramFiles}\7-Zip"
         "${env:ProgramFiles(x86)}\7-Zip"
+        "${env:ProgramW6432}\7-Zip"
         "${env:ChocolateyInstall}\bin"
         "${env:ChocolateyInstall}\tools"
         "$binDir"