ci: fix passing simple values to step parameters

In c6add4f while adding support for
more complex parameters we broke passing simple strings.

While it can be worked around by quoting the string with '', that makes
the simple case more complex than it was before, and breaks previous
pipeline definitions.

Fix it so if the first character does not look like a python expression,
take the value verbatim.

Fixes: c6add4f

Signed-off-by: Pablo Barbáchano <[email protected]>

.buildkite/common.py

@@ -139,7 +139,7 @@ def __call__(self, parser, namespace, value, option_string=None):
         res = getattr(namespace, self.dest, {})
         key_str, val = value.split("=", maxsplit=1)
         keys = key_str.split("/")
-        update = {keys[-1]: ast.literal_eval(val)}
+        update = {keys[-1]: ast.literal_eval(val) if val[0] in "[{'" else val}
         for key in list(reversed(keys))[1:]:
             update = {key: update}
         res = overlay_dict(res, update)