Avoid setting $FZF_DEFAULT_COMMAND

So that it's not propagated to the child processes and affect the
behavior of fzf started by them.

fzf 0.41.0 or above is required as it fixed the issue where
'become' process is not given a proper tty device.

Close #3299

README.md

@@ -630,8 +630,8 @@ more details.
 #### 1. Update the list of processes by pressing CTRL-R
 
 ```sh
-FZF_DEFAULT_COMMAND='ps -ef' \
-  fzf --bind 'ctrl-r:reload(eval "$FZF_DEFAULT_COMMAND")' \
+ps -ef |
+  fzf --bind 'ctrl-r:reload(ps -ef)' \
       --header 'Press CTRL-R to reload' --header-lines=1 \
       --height=50% --layout=reverse
 ```
@@ -653,12 +653,12 @@ expression `{q}`. Also, note that we used `--disabled` option so that fzf
 doesn't perform any secondary filtering.
 
 ```sh
-INITIAL_QUERY=""
-RG_PREFIX="rg --column --line-number --no-heading --color=always --smart-case "
-FZF_DEFAULT_COMMAND="$RG_PREFIX '$INITIAL_QUERY'" \
-  fzf --bind "change:reload:$RG_PREFIX {q} || true" \
-      --ansi --disabled --query "$INITIAL_QUERY" \
-      --height=50% --layout=reverse
+: | rg_prefix='rg --column --line-number --no-heading --color=always --smart-case' \
+    fzf --bind 'start:reload:$rg_prefix ""' \
+        --bind 'change:reload:$rg_prefix {q} || true' \
+        --bind 'enter:become(vim {1} +{2})' \
+        --ansi --disabled \
+        --height=50% --layout=reverse
 ```
 
 If ripgrep doesn't find any matches, it will exit with a non-zero exit status,
@@ -666,7 +666,7 @@ and fzf will warn you about it. To suppress the warning message, we added
 `|| true` to the command, so that it always exits with 0.
 
 See ["Using fzf as interactive Ripgrep launcher"](https://github.com/junegunn/fzf/blob/master/ADVANCED.md#using-fzf-as-interactive-ripgrep-launcher)
-for a fuller example with preview window options.
+for more sophisticated examples.
 
 ### Preview window