core: assign TTY to PAM context when TTYPath= is specified

Fixes #38486 again, which was fixed by
1405d46bf9, but regressed again by
f875a8026e.

This also make it use exec_input_is_terminal().

Follow-up for f875a8026e.

src/core/exec-invoke.c

@@ -1237,7 +1237,10 @@ static int exec_context_get_tty_for_pam(const ExecContext *context, char **ret)
                 return 1;
         }
 
-        if (!IN_SET(context->std_input, EXEC_INPUT_TTY, EXEC_INPUT_TTY_FAIL, EXEC_INPUT_TTY_FORCE)) {
+        /* Do not implicitly configure TTY unless TTYPath= or StandardInput=tty is specified. See issue
+         * #39334. Note, exec_context_tty_path() returns "/dev/console" when TTYPath= is unspecified, hence
+         * explicitly check context->tty_path here. */
+        if (!context->tty_path && !exec_input_is_terminal(context->std_input)) {
                 *ret = NULL;
                 return 0;
         }