env.test: fix new test for toybox echo.

The test relied on the non-builtins not being toybox (or another
multicall binary). A multicall binary will actually look at the
symlinked name (in this case "true") and do what it says on the tin.

Use a tiny shell script instead.
diff --git a/tests/env.test b/tests/env.test
index 63b9094..00b5654 100755
--- a/tests/env.test
+++ b/tests/env.test
@@ -23,6 +23,8 @@
 testcmd "replace" "A=foo PATH= `which printenv` A" "foo\n" "" ""
 # env bypasses shell builtins
-ln -s "$(which echo)" true
+echo "#!$(which sh)
+echo \$@" > true
+chmod a+x true
 testcmd "norecurse" 'env PATH="$PWD:$PATH" true hello' "hello\n" "" ""
 rm true