fix(template): fall back to $::XCATROOT when XCATROOT is not in %ENV

envvar() resolves a template variable purely from %ENV, so a template that
references $XCATROOT expands to an empty string whenever XCATROOT is not
present in the environment of the process doing the substitution. $::XCATROOT
is set as a package global at module load and is reliably available, so use it
for that one variable before falling back to %ENV.

Recovered from the unmerged lenovobuild branch (original da72c6b8).

Co-authored-by: Jarrod Johnson <10814490+jjohnson42@users.noreply.github.com>

xCAT-server/lib/perl/xCAT/Template.pm

@@ -1617,6 +1617,10 @@ sub envvar
         $envvar =~ s/^\$//;
     }
 
+    # $XCATROOT is reliably set as a package global even when it is absent from
+    # %ENV, so a template referencing it does not expand to undef.
+    if ($envvar eq 'XCATROOT') { return $::XCATROOT; }
+
     return ($ENV{$envvar});
 }