zsh: cache brew --prefix output

According to hyperfine, this shaved off ~45–50ms on my personal machine.
I expect it will be a more significant savings on my work machine based
on watching `zsh -x exit`.

    λ hyperfine 'zsh -c exit'
    Benchmark 1: zsh -c exit
      Time (mean ± σ):     195.4 ms ±   2.1 ms    [User: 98.4 ms, System: 72.8 ms]
      Range (min … max):   191.8 ms … 199.2 ms    15 runs
    # apply change
    λ hyperfine 'zsh -c exit'
    Benchmark 1: zsh -c exit
      Time (mean ± σ):     148.6 ms ±   2.2 ms    [User: 82.4 ms, System: 50.8 ms]
      Range (min … max):   144.4 ms … 152.4 ms    19 runs

It's interesting to me how much the System cost came down.

links/zshenv

@@ -133,7 +133,7 @@ fi
 
 # brew paths
 if "$has_brew" ; then
-  brew_prefix="$(brew --prefix)"
+  brew_prefix="$(cat ~/.brew-prefix 2>/dev/null || brew --prefix | tee ~/.brew-prefix)"
   pathadd_front "$brew_prefix/bin"
   pathadd_front man "$brew_prefix/share/man"
   pathadd_front info "$brew_prefix/share/info"