Put GNU grep and tar first on $PATH

GNU tar was added to the Brewfile in commit ba8623e. Commit 873df71
later modified `.zshrc` to put GNU binaries first on `$PATH` so that
the GNU programs are used instead of the macOS BSD defaults.

Commit 873df71 missed GNU `grep` and `tar`. This commit will put them
first on `$PATH`.

.zshrc

@@ -66,14 +66,16 @@ else
 fi
 TTY=$(tty)
 GNU_FINDUTILS_BIN_DIR=$HOMEBREW_PREFIX/opt/findutils/libexec/gnubin
+GNU_GREP_BIN_DIR=$HOMEBREW_PREFIX/opt/grep/libexec/gnubin
 GNU_SED_BIN_DIR=$HOMEBREW_PREFIX/opt/gsed/libexec/gnubin
+GNU_TAR_BIN_DIR=$HOMEBREW_PREFIX/opt/gnu-tar/libexec/gnubin
 PIPX_BIN_DIR=$HOME/.local/bin
 export \
   EDITOR=$editor \
   GIT_EDITOR=$editor \
   GPG_TTY=$TTY \
   HOMEBREW_NO_ANALYTICS=1 \
-  PATH=$PIPX_BIN_DIR:$GNU_FINDUTILS_BIN_DIR:$GNU_SED_BIN_DIR:$PATH \
+  PATH=$PIPX_BIN_DIR:$GNU_FINDUTILS_BIN_DIR:$GNU_GREP_BIN_DIR:$GNU_SED_BIN_DIR:$GNU_TAR_BIN_DIR:$PATH \
   PIPX_BIN_DIR=$PIPX_BIN_DIR
 
 ### aliases