aglsetup: Fix append_fragment fails with heredocs on Ubuntu 21.04.

aglsetup: Fix append_fragment fails with heredocs on Ubuntu 21.04.

Looks like bash creates temporary file for heredocs and passes fd to
deleted file to command that use heredocs. Therefore, it is better not
to use '[[ -f /dev/stdin ]]' in case of heredocs.

Replaced it with more relaxed condition.

Bug-AGL: SPEC-3990
Change-Id: Ib68d1510dd6899b0f60b207071b222ef1933bf77
Signed-off-by: vasyl <vasyl.vavrychuk@opensynergy.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26428
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>