pw_python: setup.sh arg spaces bug

Need a space for a -r arg to work. Doesn't seem to be an issue locally
but has issues when doing automated setup.

Change-Id: Idef85074afa848b6882a0ceb5709a9a2ae0b9df6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/163510
Reviewed-by: Tri Pho <tripho@google.com>
Reviewed-by: Rob Mohr <mohrr@google.com>
Pigweed-Auto-Submit: Jordan Brauer <jtbrauer@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/pw_build/python_dist/setup.sh b/pw_build/python_dist/setup.sh
index 3510bc4..cd3e2e8 100755
--- a/pw_build/python_dist/setup.sh
+++ b/pw_build/python_dist/setup.sh
@@ -30,12 +30,12 @@
 
 CONSTRAINTS_ARG=""
 if [ -f "${CONSTRAINTS_PATH}" ]; then
-    CONSTRAINTS_ARG="-c""${CONSTRAINTS_PATH}"
+    CONSTRAINTS_ARG="-c ""${CONSTRAINTS_PATH}"
 fi
 
 EXTRA_REQUIREMENT_ARG=""
 if [ -f "${EXTRA_REQUIREMENT_PATH}" ]; then
-    EXTRA_REQUIREMENT_ARG="-r""${EXTRA_REQUIREMENT_PATH}"
+    EXTRA_REQUIREMENT_ARG="-r ""${EXTRA_REQUIREMENT_PATH}"
 fi
 
 PY_MAJOR_VERSION=$(${PY_TO_TEST} -c "import sys; print(sys.version_info[0])")