Don’t fail to install magic if SHELLRC no exist

Author: mxcl

README.md

@@ -3,7 +3,7 @@
 * [`install.sh`](./install.sh) is delivered when you `curl tea.xyz`.
 * This repository also provides the `tea` GitHub Action.
 
-# GitHub Action 0.15.0
+# GitHub Action 0.15.1
 
 ```yaml
 - uses: teaxyz/setup@v0

install.sh

@@ -443,7 +443,7 @@ check_shell_magic() {
 		return 1
 	esac
 
-	if command -v grep >/dev/null 2>&1 && grep --fixed-strings "$__TEA_ONE_LINER" "$__TEA_SH_FILE" --silent; then
+	if test -f "__TEA_SH_FILE" && command -v grep >/dev/null && grep --fixed-strings "$__TEA_ONE_LINER" "$__TEA_SH_FILE" --silent; then
 		# shell magic already installed
 		return 0
 	fi