activate.fish: update fish major version check

Use fish's `string sub` command to get fish's major version from
`$FISH_VERSION`.

The `head` program in OpenBSD does not have the `-c` flag.

[main-head]: https://man.openbsd.org/head

Author: r5d

src/virtualenv/activation/fish/activate.fish

@@ -18,7 +18,7 @@ function deactivate -d 'Exit virtualenv mode and return to the normal environmen
     # reset old environment variables
     if test -n "$_OLD_VIRTUAL_PATH"
         # https://github.com/fish-shell/fish-shell/issues/436 altered PATH handling
-        if test (echo $FISH_VERSION | head -c 1) -lt 3
+        if test (string sub -s 1 -l 1 $FISH_VERSION) -lt 3
             set -gx PATH (_fishify_path "$_OLD_VIRTUAL_PATH")
         else
             set -gx PATH $_OLD_VIRTUAL_PATH
@@ -61,7 +61,7 @@ deactivate nondestructive
 set -gx VIRTUAL_ENV __VIRTUAL_ENV__
 
 # https://github.com/fish-shell/fish-shell/issues/436 altered PATH handling
-if test (echo $FISH_VERSION | head -c 1) -lt 3
+if test (string sub -s 1 -l 1 $FISH_VERSION) -lt 3
     set -gx _OLD_VIRTUAL_PATH (_bashify_path $PATH)
 else
     set -gx _OLD_VIRTUAL_PATH $PATH