.zshrc: Explicitly add /usr/local/bin to PATH

The macOS Terminal.app gets this automatically, but when you run other
terminal emulators like Alacritty it doesn't for some reason.

.zshrc

@@ -21,6 +21,9 @@ compinit
 
 # OS-specific things
 if [[ "$OSTYPE" =~ ^darwin.*$ ]]; then
+    # explicitly add Homebrew binary paths just in case Terminal doesn't inherit it from macOS, for example Alacritty.
+    PATH=$PATH:/usr/local/bin
+
     # Add brew stuff to MANPATH
     MANPATH=/usr/local/share/man:/usr/local/opt/postgresql@9.5/share/man:$PATH