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

[tobik]

Summary

• The wrapper scripts leave the shell running for no reason. Add an exec to replace the shell process
• While here use ECHO_CMD not ECHO (ECHO is set to true on make -s in /usr/share/mk/sys.mk and shouldn't be used for things like this)
  
  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 -

[arrowd]

Thanks for this, it is indeed a good change.