[PATCH] fix(build): fix builds with interactive manpagers

[Max Coplan <mchcopl@xxxxxxxxx>]

Summary:

The `MANPAGER` on my machine is `nvim`.  When running the `man` stage of the build, the build would freeze without giving any feedback.  This commit manually sets the manpager to the noninteractive `cat`.  This fixes the build.

Test Plan:
- Run `MANPAGER="nvim +Man!" make`
before: build would hang at `attempting man`
now: build completes successfully

PR: https://github.com/zsh-users/zsh/pull/132 

patch: https://patch-diff.githubusercontent.com/raw/zsh-users/zsh/pull/132.patch

diff: https://patch-diff.githubusercontent.com/raw/zsh-users/zsh/pull/132.diff