Change the version check invocation in install.ps1

When install.ps1 is executed inside MSYS (or cygwin I suspect) cmd
is a shell script that invokes the real cmd.  But in install.ps1 it
tries to open this shell script instead of executing it.

AFAICT the only reason to use cmd /c is to capture stderr because
there is a bug with powersehll; but fzf prints the version information
on stdout so we don't need to care about stderr.  So we can just call
the binary directly.  This avoids the cmd issues.

install.ps1

@@ -4,7 +4,7 @@ $fzf_base=Split-Path -Parent $MyInvocation.MyCommand.Definition
 
 function check_binary () {
   Write-Host "  - Checking fzf executable ... " -NoNewline
-  $output=cmd /c $fzf_base\bin\fzf.exe --version 2>&1
+  $output= & $fzf_base\bin\fzf.exe --version
   if (-not $?) {
     Write-Host "Error: $output"
     $binary_error="Invalid binary"