KRB5:`do_keytab_copy()`: don't `faccessat()` for types other than 'FILE:'

[257]

Introduced in 3d27526
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The readability check with faccessat is applied to all keytab types that are not MEMORY:. This could cause issues with other valid keytab types like KCM: or API:, which are not file-based and for which faccessat would likely fail, causing a regression.

To avoid this, the check should be performed only for FILE: keytabs.

    if (strncmp(keytab_name, "FILE:", sizeof("FILE:") - 1) == 0) {
        if (faccessat(AT_FDCWD, sep+1, R_OK, AT_EACCESS) != 0) {
            saved_errno = errno;
            DEBUG(SSSDBG_CRIT_FAILURE,
                  "keytab [%s] is not readable: [%d][%s].\n",
                  keytab_file, saved_errno, sss_strerror(saved_errno));

            kerr = KRB5KRB_ERR_GENERIC;
            goto done;
        }
    }

Originally posted by @gemini-code-assist[bot] in #8554 (comment)