ci: suggest setting PMBOOTSTRAP_CMD when unable to locate pmbootstrap.py (MR 5033)

When running locally, sometimes pmbootstrap in PATH is not a symlink and
the auto-detection stuff fails. This can be resolved by setting
PMBOOTSTRAP_CMD=/path/to/pmbootstrap.py, so the error has been improved
to suggest doing this.

[ci:skip-build]: already built successfully in CI

.ci/testcases/add_pmbootstrap_to_import_path/__init__.py

@@ -34,7 +34,7 @@ def path_pmbootstrap():
             return directory
 
     # Symlink not set up properly
-    print("ERROR: 'pmbootstrap' is not a symlink to pmbootstrap.py")
+    print("ERROR: unable to find path to pmbootstrap.py. Try setting 'PMBOOTSTRAP_CMD'.")
     sys.exit(1)