share/completions/env: Fix

This completed the commandline with options removed, which looked like

    env VAR=VAL command option

Which didn't really actually work.

Fixes #5856.

[ci skip]

share/completions/env.fish

@@ -1,6 +1,20 @@
 function __fish_complete_env_subcommand
     argparse -s s/ignore-environment u/unset h-help v-version -- (commandline -opc) (commandline -ct) 2>/dev/null
     or return
+
+    # argv[1] is `env` or an alias.
+    set -e argv[1]
+
+    # Remove all VAR=VAL arguments up to the first that isn't
+    while set -q argv[1]
+        if string match -q '*=*' -- $argv[1]
+            set -e argv[1]
+        else
+            break
+        end
+    end
+
+    # Then complete the rest as if it was given as a command.
     if set -q argv[1]
         complete -C"$argv"
         return 0