Use the full path for noshebang'd scripts

If you make a script called `foo` somewhere in $PATH, and did not give
it a shebang, this would end up calling

    sh foo

instead of

    sh /usr/bin/foo

which might not match up.

Especially if the path is e.g. `--version` or `-` that would end up
being misinterpreted *by sh*.

So instead we simply pass the actual_cmd to sh, because we need it
anyway to get it to fail to execute before.

tests/checks/noshebang.fish

@@ -80,3 +80,13 @@ runfile
 
 #CHECK: 126
 #CHECKERR: exec: {{.*}}
+
+echo 'echo foo' >./-
+sleep 0.1
+chmod +x ./-
+set PATH ./ $PATH
+sleep 0.1
+-
+#CHECK: foo
+echo $status
+#CHECK: 0