Uninify MSVC version that is used in provisioning

We pick the oldest build tools we can find because there is lower
chance that the produced binaries will be incomaptible with the newer
version of MSVC.

With the current provisioning design we also cannot change the MSVC
version for the each package on demand. Once Enter-VsDevShell is called
it litters the powershell environment and blocks the use of
Enter-VsDevShell second time. It could be better to run each .ps1 file
in own instance of powershell to avoid mixing build environment.

Amends f58afd5476

Pick-to: 6.5
Change-Id: Ie752cfc8b69ed985e61ec16209007dd586611866
Reviewed-by: Samuel Mira <samuel.mira@qt.io>
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>

coin/provisioning/common/windows/install-ffmpeg.ps1

@@ -107,14 +107,8 @@ function InstallMingwFfmpeg {
 
 
 function InstallMsvcFfmpeg {
-    $vsPath = GetVSPath
-
-    Write-Host "Enter VisualStudio developer shell"
-    try {
-        Import-Module "$vsPath\Common7\Tools\Microsoft.VisualStudio.DevShell.dll"
-        Enter-VsDevShell -VsInstallPath $vsPath -DevCmdArguments "-arch=x64 -no_logo"
-    } catch {
-        Write-Host "Failed to enter VisualStudio DevShell"
+    $result = EnterVSDevShell
+    if (-Not $result) {
         return $false
     }