fix(ci): do not crash if file in repo root is modified

by unconditionally grabbing the second part of the path of a modified
file in `run_all_tests()`, we ended up indexing out of bounds if a
modified file does not _have_ a second component in its path (e.g. if
the file is at the repository root, like `Cargo.lock`). Fix this by
checking for the length of x.parts first, and using python's
short-circuiting behavior of logical operators.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

.buildkite/common.py

@@ -123,7 +123,7 @@ def run_all_tests(changed_files):
     return not changed_files or any(
         x.suffix != ".md"
         and not (x.parts[0] == ".github" and x.suffix == ".yml")
-        and x.parts[1] != "hiding_ci"
+        and (len(x.parts) < 2 or x.parts[1] != "hiding_ci")
         for x in changed_files
     )