Improve guard so it works for no eof new line

compiler/nix-build.vim

@@ -1,4 +1,5 @@
 if !exists('g:polyglot_disabled') || index(g:polyglot_disabled, 'nix') == -1
+
 if exists('current_compiler')
     finish
 endif
@@ -10,4 +11,5 @@ endif
 
 CompilerSet errorformat=error:\ %m\ at\ %f:%l:%c,builder\ for\ \'%m\'\ failed\ with\ exit\ code\ %n,fixed-output\ derivation\ produced\ path\ \'%s\'\ with\ %m
 CompilerSet makeprg=nix-build
+
 endif