Mk/Uses/cabal.mk: Replace shell process in EXECUTABLES wrapper scripts

The wrapper scripts leave the shell running for no reason. Add an
exec to replace the shell process.

Before:
$ ps -d
35230 5 Ss 0:00.09 -oksh (oksh)
35351 5 S+ 0:00.00 - /bin/sh /usr/local/bin/shellcheck -
35352 5 S+ 0:00.00 `-- /usr/local/libexec/cabal/shellcheck -

After:
$ ps -d
35230 5 Is 0:00.05 -oksh (oksh)
35236 5 I+ 0:00.01 - /usr/local/libexec/cabal/shellcheck -

Approved by: arrowd
Differential Revision: https://reviews.freebsd.org/D29852