fix: respect user-defined PIPX_BIN_DIR

[skycaptain]

If pipx is already installed in the base image, it's likely that PIPX_BIN_DIR is set to a custom location such as /usr/local/bin. Since this script doesn't yet support --global, this at least adds support for user-defined PIPX_BIN_DIRs. Without this support, the script would create broken symlinks, e.g. when installing ghcr.io/prulloac/devcontainer-features/pre-commit:1 and PIPX_BIN_DIR=/usr/local/bin and PIPX_HOME=/opt/pipx, pipx installs the pre-commit symlink to /opt/pipx/bin/pre-commit, which results in a broken symlink:

root ➜ / $ which pre-commit
root ➜ / $ pipx list
venvs are in /opt/pipx/venvs
apps are exposed on your $PATH at /usr/local/bin
   package pre-commit 3.8.0, installed using Python 3.11.2
    - pre-commit (symlink missing or pointing to unexpected location)