wrapFish: fix early variable expansion

the bash script will retrieve the $var and cause the variable expand to empty
add \ to prevent this, and let fish command to retrieve the variable

pkgs/shells/fish/wrapper.nix

···
     set --prepend fish_complete_path ${escapeShellArgs complPath}
     set --prepend fish_function_path ${escapeShellArgs funcPath}
     set --local fish_conf_source_path ${escapeShellArgs confPath}
-    for c in $fish_conf_source_path/*; source $c; end
+    for c in \$fish_conf_source_path/*; source \$c; end
   " "$@"
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