make .ps1 shims work the same as .cmd shims

lib/core.ps1

@@ -89,7 +89,7 @@ function shim($path, $global) {
 	# note: use > for first line to replace file, then >> to append following lines
 	echo '# ensure $HOME is set for MSYS programs' > $shim
 	echo "if(!`$env:home) { `$env:home = `"`$home\`" }" >> $shim
-	echo 'if($env:home -eq "\") { $env:home = $null } # running as system' >> $shim
+	echo 'if($env:home -eq "\") { $env:home = $env:allusersprofile }' >> $shim
 	echo "`$path = '$path'" >> $shim
 	echo 'if($myinvocation.expectingInput) { $input | & $path @args } else { & $path @args }' >> $shim