Fix regex used for mangling druz/DNSKEY

Fix regex used for mangling druz/DNSKEY

During "dlv" system test setup, the "sed" regex used for mangling the
DNSKEY RRset for the "druz" zone does not include the plus sign ("+"),
which may:

  - cause the replacement to happen near the end of DNSKEY RDATA, which
    can cause the latter to become an invalid Base64 string,

  - prevent the replacement from being performed altogether.

Both cases prevent the "dlv" system test from behaving as intended and
may trigger false positives.  Add the missing character to the
aforementioned regex to ensure the replacement is always performed on
bytes 10-25 of DNSKEY RDATA.

(cherry picked from commit fd13fef2999767e71e4aa4847023efac20f3e789)