Use qualified output path for -share flag

[EmeraldShift]

Fix a bug with -share where the share command was passed only the basename of the file to share, by using ~output_path instead of ~filename.

At tip, I can run

MAGIC_TRACE_SHARE_DOC=" Share the trace." \
MAGIC_TRACE_SHARE_COMMAND=/usr/bin/echo \
dune exec bin/magic_trace_bin.exe -- run true -o /tmp/true.fxt -share

and I no longer receive an exception, and the final output is /tmp/true.fxt.

[Xyene]

LGTM, thanks!