nukeReferences: fix for store paths with special characters (like '+' or '.')

This introduces extra escaping for $NIX_STORE that gets interpolated
into a PCRE. The escaping is performed using a standard Perl function
"quotemeta" (see "perldoc -f quotemeta" for reference). The same value
is also used in sed regex which uses POSIX basic regular expressions
instead of PCRE, so it needs fewer characters to be escaped. It should
not cause much problem to not change sed invocation, but I replace it
with equivalent Perl expression (actually the behavior is changed to not
output a newline character after the matched output).