completions/env: Fix errant loud string match

This was vexing me for a while because the extraneous output presented as a
valid (but unwanted) completion, i.e. with RUSTC_WRAPPER exported, `env RUSTC_W`
would offer `RUSTC_W=` and `RUSTC_WRAPPER=` as completions (when only the latter
should have been offered up).

share/completions/env.fish

@@ -25,7 +25,7 @@ function __fish_env_redefine_vars
 
     set cmdline "$(commandline -xp)"
     for var in $vars
-        if not string match -e -- $var= $cmdline
+        if not string match -eq -- $var= $cmdline
             echo $var=
         end
     end