pipenv: fix error when Pipfile is not a file (#8931)

plugins/pipenv/pipenv.plugin.zsh

@@ -7,7 +7,7 @@ compdef _pipenv pipenv
 # Automatic pipenv shell activation/deactivation
 _togglePipenvShell() {
   # deactivate shell if Pipfile doesn't exist and not in a subdir
-  if [[ ! -a "$PWD/Pipfile" ]]; then
+  if [[ ! -f "$PWD/Pipfile" ]]; then
     if [[ "$PIPENV_ACTIVE" == 1 ]]; then
       if [[ "$PWD" != "$pipfile_dir"* ]]; then
         exit
@@ -17,7 +17,7 @@ _togglePipenvShell() {
 
   # activate the shell if Pipfile exists
   if [[ "$PIPENV_ACTIVE" != 1 ]]; then
-    if [[ -a "$PWD/Pipfile" ]]; then
+    if [[ -f "$PWD/Pipfile" ]]; then
       export pipfile_dir="$PWD"
       pipenv shell
     fi