Along the way, remove the get_envvar() function, as it was just a
convenient wrapper for a two step process that we no longer use.

This follows up to commit 9b10692a, which had effectively added a
requirement that the user set PASSWORD_STORE_EXTENSIONS_DIR.  We don't
actually need the user to do that; we just need to be able to find the
extensions directory so we can check things in it.  See this comment:
9b10692a (comment 7757)