Change hostname variable name to avoid scope error

Add check for existing fish `hostname` variable

functions/_pure_prompt_host.fish

@@ -1,6 +1,7 @@
 function _pure_prompt_host
-    set --local hostname (hostname -s) # current host name
+    set -qg hostname
+    or set --local hostname (hostname -s) # current host name
     set --local hostname_color "$pure_host_color"
 
     echo "$hostname_color$hostname"
-end
+end