Fixing python benchmark continuous test (#10420)

* Fixing bad syntax

* Revert runtime fix

* Second try at benchmarks

kokoro/linux/benchmark/run.sh

@@ -12,11 +12,13 @@ set -ex
 export OUTPUT_DIR=testoutput
 repo_root="$(pwd)"
 
-# Setup python virtual environment.
-apt-get update -y
-apt-get install -y --no-install-recommends python3 python3-venv
-python3 -m venv venv
-source venv/bin/activate
+# Setup python environment.
+pyenv install -v 3.9.5 -s
+pyenv global 3.9.5
+pyenv versions
+python --version
+python -m venv "venv"
+source "venv/bin/activate"
 
 # TODO(jtattermusch): Add back support for benchmarking with tcmalloc for C++ and python.
 # This feature was removed since it used to use tcmalloc from https://github.com/gperftools/gperftools.git