[tasktool] Install default lit from pip

For some reason, even though lit is *installed*, it doesn't actually work when
you install the latest version via pip.

For the sake of keeping any LNT-based bots green, let's just use the lit that
you can get off pip. We were using a stale version for months anyway, so
it's not like we're making anything objectively worse.

tasks/utils/venv_lit.sh

@@ -1,4 +1,3 @@
 echo "@@@ Install VirtualEnv Lit @@@"
-svn checkout -q --trust-server-cert-failures=unknown-ca --non-interactive https://github.com/llvm/llvm-project/trunk/llvm/utils/lit/ lit-src
-. "${TASKDIR}"/utils/pip_install.sh lit-src/
+. "${TASKDIR}"/utils/pip_install.sh lit
 echo "@@@@@@"