Place Z3 in PATH

[Munken]

The synth:formal image ships with the Z3 solver, but it is not in the PATH

docker run -it ghdl/synth:formal bash
root@7be5976e3a3a:/# z3
bash: z3: command not found

The location of the binary is /opt/z3/z3/bin/z3, but this is not in the PATH:

root@7be5976e3a3a:/# echo $PATH
/opt/ghdl/bin:/opt/yosys/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

This PR adds the Z3 bin dir to the path.

Also not sure it is opt/z3/z3 when ghdl is simply placed in /opt/ghdl.

[eine]

Actually, the COPY command is not correct. It should be COPY --from=ghdl/cache:formal ./z3 /opt/z3. Would you find fixing it and modifying the PATH accordingly?

[Munken]

COPY command and PATH adjusted.

[eine]

Thanks!