Disable extern file check for venv activation in script

Suggests that some refactoring is in order, but since this is also testing
shellcheck itself, it's an OK exercise. Since there isn't a venv created in the
CI action that runs shellcheck, it can't read the script to check for issues.

Author: steve-downey

scripts/install_run_precommit.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# shellcheck disable=SC1091
 
 python3 -m venv .venv