[AUR] A few fixes to the update-aur.sh script

Summary:

• The updpkgsums script is expected to replace the sums using the algorithm already declared in the PKGBUILD. The update-aur.sh script actually does this by declaring the sums as sha256 in the PKGBUILD file, but this is not enough to trigger the detection by updpkgsums. This diff fixes this by adding a dummy hash value.
• The cp -R "${PACKAGE}"/* "${PACKAGE}-upstream" glob expansion will not copy the .SRCINFO file due to it starting with a dot. This diff fixes the copy syntax to include all files including the hidden ones.

Test Plan:

./update-aur.sh bitcoin-abc

Check the checksum is using sha256 as expected.
Check the .SRCINFO file is part of the commit.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7306