[xcelium] Pass cov_merge_db_dir through to cov_report.tcl
[xcelium] Pass cov_merge_db_dir through to cov_report.tcl

This was removed by 6e5fc98, which changed the script to not use the
variable any more. Unfortunately, that commit was closely followed by
0012ec3 which used the variable. (Looking at the dates, I think this
was probably merge skew from a slow PR review)

Put it back!

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan