pmb: Set PMBOOTSTRAP_CMD to argv[0] in environment

This allows for scripts run by pmbootstrap to figure out how pmbootstrap
was invoked, and with that invoke pmbootstrap even in unconventional
setups where "pmbootstrap" isn't an available command in $PATH.

Reviewed-by: Oliver Smith <ollieparanoid@postmarketos.org>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20230325205337.105795-1-newbyte@postmarketos.org%3E

pmb/__init__.py

@@ -23,6 +23,9 @@ def main():
         args = parse.arguments()
         os.umask(0o22)
 
+        # Store script invocation command
+        os.environ["PMBOOTSTRAP_CMD"] = sys.argv[0]
+
         # Sanity checks
         other.check_grsec()
         if not args.as_root and os.geteuid() == 0: