installworld: improve portability of ldd use

b3b462229f97 added a case statement to ignore lines containing strings
in square brackets such as "[vdso]" and "[preloaded]". On MacOS
Monterey where /bin/sh may be zsh, this fails with:

/bin/sh: -c: line 0: syntax error near unexpected token `;;'

Invoke grep in the pipeline to remove such lines instead.

Reviewed by: emaste
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D35618