feat: provide YASHCHIKI_HOME

This allows for env-var-based setting of `--caches-dir=…` — useful for
CI.

Change-Id: I704da984385c0bbc4b31d5bb1d756c6b1104e4b7

bin/yashchiki

@@ -75,15 +75,21 @@ parser.add_argument(
 # optional with persistent default
 parser.add_argument(
     "--caches-dir", type=pathlib.Path,
-    default=os.path.expanduser("~/.yashchiki/"),
+    default="YASHCHIKI_HOME" in os.environ and
+        os.environ["YASHCHIKI_HOME"] or
+        os.path.expanduser("~/.yashchiki/"),
     help="Location of caches to use.")
 parser.add_argument(
     "--log-dir", type=pathlib.Path,
-    default=os.path.expanduser("~/.yashchiki/log/"),
+    default="YASHCHIKI_HOME" in os.environ and
+        pathlib.Path(os.environ["YASHCHIKI_HOME"] + "/log") or
+        os.path.expanduser("~/.yashchiki/log/"),
     help="Location of logs to use.")
 parser.add_argument(
     "--sandboxes-dir", type=pathlib.Path,
-    default=os.path.expanduser("~/.yashchiki/sandboxes"),
+    default="YASHCHIKI_HOME" in os.environ and
+        pathlib.Path(os.environ["YASHCHIKI_HOME"] + "/sandboxes") or
+        os.path.expanduser("~/.yashchiki/sandboxes"),
     help="Location of sandboxes for container creation to use.")
 # optional with temporary default
 parser.add_argument(