bootstrap.py: let --version flag take precedence over env vars

bootstrap/bootstrap.py

@@ -364,7 +364,7 @@ def main():
     )
     parser.add_argument(
         "--version",
-        default="latest",
+        default="",
         help=(
             "TLJH version or Git reference. "
             "Default 'latest' is the most recent release. "
@@ -478,21 +478,26 @@ def main():
     logger.info("Upgrading pip...")
     run_subprocess([hub_env_pip, "install", "--upgrade", "pip"])
 
-    # Install/upgrade TLJH installer
+    # pip install TLJH installer based on
+    #
+    #   1. --version, _resolve_git_version is used
+    #   2. TLJH_BOOTSTRAP_PIP_SPEC (then also respect TLJH_BOOTSTRAP_DEV)
+    #   3. latest, _resolve_git_version is used
+    #
     tljh_install_cmd = [hub_env_pip, "install", "--upgrade"]
-    if os.environ.get("TLJH_BOOTSTRAP_DEV", "no") == "yes":
-        logger.info("Selected TLJH_BOOTSTRAP_DEV=yes...")
-        tljh_install_cmd.append("--editable")
-
     bootstrap_pip_spec = os.environ.get("TLJH_BOOTSTRAP_PIP_SPEC")
-    if not bootstrap_pip_spec:
+    if args.version or not bootstrap_pip_spec:
+        version_to_resolve = args.version or "latest"
         bootstrap_pip_spec = (
             "git+https://github.com/jupyterhub/the-littlest-jupyterhub.git@{}".format(
-                _resolve_git_version(args.version)
+                _resolve_git_version(version_to_resolve)
             )
         )
-
+    elif os.environ.get("TLJH_BOOTSTRAP_DEV", "no") == "yes":
+        logger.info("Selected TLJH_BOOTSTRAP_DEV=yes...")
+        tljh_install_cmd.append("--editable")
     tljh_install_cmd.append(bootstrap_pip_spec)
+
     if initial_setup:
         logger.info("Installing TLJH installer...")
     else: