LP#1793585: Fix Unescaped left brace in regex Perl error.

LP#1793585: Fix Unescaped left brace in regex Perl error.

Running make check results in the following error on Perl 5.26:

Unescaped left brace in regex is illegal here in regex; marked by <-- HERE in m/\${ <-- HERE EMAIL_SENDER}/ at Open-ILS/src/perlmods/lib/OpenILS/Application/Circ/HoldNotify.pm line 358.

Prior to Ubuntu 18.04 and Perl 5.26, this was a deprecation warning.
This commit fixes the regular expressions in HoldNotify.pm to escape
the left braces to make this error/warning go away.

Signed-off-by: Jason Stephenson <jason@sigio.com>
Signed-off-by: Ben Shum <ben@evergreener.net>