Check for venv existence before creating in justfile (#8)

Author: derek-globus

justfile

@@ -5,7 +5,9 @@ help:
 
 # Install a development virtual environment (at `./.venv`).
 install:
-    python -m venv .venv
+    if [ ! -d .venv ]; then
+        python -m venv .venv
+    fi
     .venv/bin/pip install --upgrade pip setuptools wheel
     .venv/bin/pip install -e.
     .venv/bin/pip install -r requirements/test/requirements.txt