Derive pipx's venv directory automatically

The directory's location can differ across systems. Fedora places it at
`~/.local/share/pipx/venvs/` while macOS places it at
`~/.local/pipx/venvs/`.

README.md

@@ -30,13 +30,13 @@ pipx install --python=3.12 --fetch-missing-python standardebooks
 
 ```shell
 # Install ZSH completions.
-sudo ln -s $HOME/.local/pipx/venvs/standardebooks/lib/python3.*/site-packages/se/completions/zsh/_se /usr/share/zsh/vendor-completions/_se && hash -rf && compinit
+sudo ln -s $(pipx environment --value PIPX_LOCAL_VENVS)/standardebooks/lib/python3.*/site-packages/se/completions/zsh/_se /usr/share/zsh/vendor-completions/_se && hash -rf && compinit
 
 # Install Bash completions.
-sudo ln -s $HOME/.local/pipx/venvs/standardebooks/lib/python3.*/site-packages/se/completions/bash/se /usr/share/bash-completion/completions/se
+sudo ln -s $(pipx environment --value PIPX_LOCAL_VENVS)/standardebooks/lib/python3.*/site-packages/se/completions/bash/se /usr/share/bash-completion/completions/se
 
 # Install Fish completions.
-ln -s $HOME/.local/pipx/venvs/standardebooks/lib/python3.*/site-packages/se/completions/fish/se $HOME/.config/fish/completions/se.fish
+ln -s $(pipx environment --value PIPX_LOCAL_VENVS)/standardebooks/lib/python3.*/site-packages/se/completions/fish/se $HOME/.config/fish/completions/se.fish
 ```
 
 ## Fedora 41 users
@@ -57,13 +57,13 @@ pipx inject standardebooks setuptools
 
 ```shell
 # Install ZSH completions.
-sudo ln -s $HOME/.local/share/pipx/venvs/standardebooks/lib/python3.*/site-packages/se/completions/zsh/_se /usr/share/zsh/vendor-completions/_se && hash -rf && compinit
+sudo ln -s $(pipx environment --value PIPX_LOCAL_VENVS)/standardebooks/lib/python3.*/site-packages/se/completions/zsh/_se /usr/share/zsh/vendor-completions/_se && hash -rf && compinit
 
 # Install Bash completions.
-sudo ln -s $HOME/.local/share/pipx/venvs/standardebooks/lib/python3.*/site-packages/se/completions/bash/se /usr/share/bash-completion/completions/se
+sudo ln -s $(pipx environment --value PIPX_LOCAL_VENVS)/standardebooks/lib/python3.*/site-packages/se/completions/bash/se /usr/share/bash-completion/completions/se
 
 # Install Fish completions.
-ln -s $HOME/.local/share/pipx/venvs/standardebooks/lib/python3.*/site-packages/se/completions/fish/se $HOME/.config/fish/completions/se.fish
+ln -s $(pipx environment --value PIPX_LOCAL_VENVS)/standardebooks/lib/python3.*/site-packages/se/completions/fish/se $HOME/.config/fish/completions/se.fish
 ```
 
 ## macOS users
@@ -86,10 +86,10 @@ ln -s $HOME/.local/share/pipx/venvs/standardebooks/lib/python3.*/site-packages/s
 	pipx install --python python3.11 standardebooks
 
 	# Optional: Bash users who have set up bash-completion via brew can install tab completion.
-	ln -s $HOME/.local/pipx/venvs/standardebooks/lib/python3.*/site-packages/se/completions/bash/se $(brew --prefix)/etc/bash_completion.d/se
+	ln -s $(pipx environment --value PIPX_LOCAL_VENVS)/standardebooks/lib/python3.*/site-packages/se/completions/bash/se $(brew --prefix)/etc/bash_completion.d/se
 
 	# Optional: Fish users can install tab completion.
-	ln -s $HOME/.local/pipx/venvs/standardebooks/lib/python3.*/site-packages/se/completions/fish/se $HOME/.config/fish/completions/se.fish
+	ln -s $(pipx environment --value PIPX_LOCAL_VENVS)/standardebooks/lib/python3.*/site-packages/se/completions/fish/se $HOME/.config/fish/completions/se.fish
 	```
 
 ## OpenBSD 6.6 Users