Add persistent storage of installed toolchains (#3797)
Extends #3726
Moves toolchain storage out of `UV_BOOTSTRAP_DIR` (`./bin`) into the
proper user data directory as defined by #3726.
Replaces `UV_BOOTSTRAP_DIR` with `UV_TOOLCHAIN_DIR` for customization.
Installed toolchains will be discovered without opt-in, but the idea is
still that these are not yet user-facing.
