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.

.github/workflows/ci.yml

@@ -144,7 +144,6 @@ jobs:
 
       - name: "Cargo test"
         run: |
-          export UV_BOOTSTRAP_DIR="$(pwd)/bin"
           cargo nextest run \
             --features python-patch \
             --workspace \