CI: Support not having "pmbootstrap" in $PATH (MR 3977)

Not everyone's setup includes "pmbootstrap" as a command in $PATH.
Currently, in such setups, $ pmbootstrap ci kconfig (and potentially
other testcases) in the pmaports repository do not work as they expect
the "pmbootstrap" command to exist. Resolve this by checking whether
PMBOOTSTRAP_CMD is set and use that as command for invoking pmbootstrap.
If it isn't set (because you're using an old pmbootstrap version or
invoking a script directly for some reason), fall back to the old
behaviour as to not break compatibility.

Depends on pmbootstrap patch that sets PMBOOTSTRAP_CMD to argv[0]:
https://lists.sr.ht/~postmarketos/pmbootstrap-devel/patches/39995

.ci/testcases/add_pmbootstrap_to_import_path/__init__.py

@@ -15,8 +15,10 @@ def path_pmbootstrap():
         code from there.
         returns: pmbootstrap installation folder
     """
-    # Find 'pmbootstrap' executable
-    bin = shutil.which("pmbootstrap")
+    # This variable is set by pmbootstrap 1.52 and later
+    # If it's undefined, try to find 'pmbootstrap' in path
+    bin = os.environ.get("PMBOOTSTRAP_CMD") or shutil.which("pmbootstrap")
+
     if not bin:
         print("ERROR: 'pmbootstrap' not found in $PATH")
         sys.exit(1)